Linux内核获取EDID流程
drivers/gpu/drm/drm_edid.c中
/**
* drm_get_edid - get EDID data, if available
* @connector: connector we're probing
* @adapter: I2C adapter to use for DDC
*
* Poke the given I2C channel to grab EDID data if possible. If found,
* attach it to the connector.
*
* Return: Pointer to valid EDID or NULL if we couldn't find any.
*/
struct edid *drm_get_edid(struct drm_connector *connector,
struct i2c_adapter *adapter)
{
struct edid *edid;
if (connector->force == DRM_FORCE_OFF)
return NULL;
if (connector->force == DRM_FORCE_UNSPECIFIED && !drm_probe_ddc(adapter))
return NULL;
edid = drm_do_get_edid(connector, drm_do_probe_ddc_edid, adapter);
if (edid)
drm_get_displayid(connector, edid);
return edid;
}
EXPORT_SYMBOL(drm_get_edid);drm_do_get_edit函数在同文件中实现，源码如下：
/**
* drm_do_get_edid - get EDID data using a custom EDID block read function
* @connector: connector we're probing
* @get_edid_block: EDID block read function
* @data: private data passed to the block read function
*
* When the I2C adapter connected to the DDC bus is hidden behind a device that
* exposes a different interface to read EDID blocks this function can be used
* to get EDID data using a custom block read function.
*
* As in the general case the DDC bus is accessible by the kernel at the I2C
* level, drivers must make all reasonable efforts to expose it as an I2C
* adapter and use drm_get_edid() instead of abusing this function.
*
* The EDID may be overridden using debugfs override_edid or firmare EDID
* (drm_load_edid_firmware() and drm.edid_firmware parameter), in this priority
* order. Having either of them bypasses actual EDID reads.
*
* Return: Pointer to valid EDID or NULL if we couldn't find any.
*/
struct edid *drm_do_get_edid(struct drm_connector *connector,
int (*get_edid_block)(void *data, u8 *buf, unsigned int block,
size_t len),
void *data)
{
int i, j = 0, valid_extensions = 0;
u8 *edid, *new;
struct edid *override;
override = drm_get_override_edid(connector);
if (override)
return override;
if ((edid = kmalloc(EDID_LENGTH, GFP_KERNEL)) == NULL)
return NULL;
/* base block fetch */
for (i = 0; i < 4; i++) {
if (get_edid_block(data, edid, 0, EDID_LENGTH))
goto out;
if (drm_edid_block_valid(edid, 0, false,
&connector->edid_corrupt))
break;
if (i == 0 && drm_edid_is_zero(edid, EDID_LENGTH)) {
connector->null_edid_counter++;
goto carp;
}
}
if (i == 4)
goto carp;
/* if there's no extensions, we're done */
valid_extensions = edid[0x7e];
if (valid_extensions == 0)
return (struct edid *)edid;
new = krealloc(edid, (valid_extensions + 1) * EDID_LENGTH, GFP_KERNEL);
if (!new)
goto out;
edid = new;
for (j = 1; j <= edid[0x7e]; j++) {
u8 *block = edid + j * EDID_LENGTH;
for (i = 0; i < 4; i++) {
if (get_edid_block(data, block, j, EDID_LENGTH))
goto out;
if (drm_edid_block_valid(block, j, false, NULL))
break;
}
if (i == 4)
valid_extensions--;
}
if (valid_extensions != edid[0x7e]) {
u8 *base;
connector_bad_edid(connector, edid, edid[0x7e] + 1);
edid[EDID_LENGTH-1] += edid[0x7e] - valid_extensions;
edid[0x7e] = valid_extensions;
new = kmalloc_array(valid_extensions + 1, EDID_LENGTH,
GFP_KERNEL);
if (!new)
goto out;
base = new;
for (i = 0; i <= edid[0x7e]; i++) {
u8 *block = edid + i * EDID_LENGTH;
if (!drm_edid_block_valid(block, i, false, NULL))
continue;
memcpy(base, block, EDID_LENGTH);
base += EDID_LENGTH;
}
kfree(edid);
edid = new;
}
return (struct edid *)edid;
carp:
connector_bad_edid(connector, edid, 1);
out:
kfree(edid);
return NULL;
}
EXPORT_SYMBOL_GPL(drm_do_get_edid);drm_do_probe_ddc_edid函数也在同文件（drivers/gpu/drm/drm_edid.c）中实现，源码如下（此时函数形参对应的实参分别是drm_get_edid函数中的struct i2c_adapter *adapter的adapter，drm_do_get_edid函数中分配的edid，0和EDID_LENGTH(值为128)）：
#define DDC_SEGMENT_ADDR 0x30
/**
* drm_do_probe_ddc_edid() - get EDID information via I2C
* @data: I2C device adapter
* @buf: EDID data buffer to be filled
* @block: 128 byte EDID block to start fetching from
* @len: EDID data buffer length to fetch
*
* Try to fetch EDID information by calling I2C driver functions.
*
* Return: 0 on success or -1 on failure.
*/
static int
drm_do_probe_ddc_edid(void *data, u8 *buf, unsigned int block, size_t len)
{
struct i2c_adapter *adapter = data;
unsigned char start = block * EDID_LENGTH;
unsigned char segment = block >> 1;
unsigned char xfers = segment ? 3 : 2;
int ret, retries = 5;
/*
* The core I2C driver will automatically retry the transfer if the
* adapter reports EAGAIN. However, we find that bit-banging transfers
* are susceptible to errors under a heavily loaded machine and
* generate spurious NAKs and timeouts. Retrying the transfer
* of the individual block a few times seems to overcome this.
*/
do {
struct i2c_msg msgs[] = {
{
.addr = DDC_SEGMENT_ADDR,
.flags = 0,
.len = 1,
.buf = &segment,
}, {
.addr = DDC_ADDR,
.flags = 0,
.len = 1,
.buf = &start,
}, {
.addr = DDC_ADDR,
.flags = I2C_M_RD,
.len = len,
.buf = buf,
}
};
/*
* Avoid sending the segment addr to not upset non-compliant
* DDC monitors.
*/
ret = i2c_transfer(adapter, &msgs[3 - xfers], xfers);
if (ret == -ENXIO) {
DRM_DEBUG_KMS("drm: skipping non-existent adapter %s\n",
adapter->name);
break;
}
} while (ret != xfers && --retries);
return ret == xfers ? 0 : -1;
}
其中，DDC_ADDR在include/drm/drm_edid.h中定义：#define DDC_ADDR 0x50，代表DDC的从设备地址；DDC_SEGMENT_ADDR在本段代码开始处定义：#define DDC_SEGMENT_ADDR 0x30。
实际上这个函数就是调用i2c_transfer函数通过i2c也可以说是DDC读取edid，将返回值保存在buf即edid指向的内存中。
i2c_transfer函数就是Linux i2c框架的接口函数了，要详细了解可以看Linux i2c相关的内容，在此不深入讲解，只是把相关代码贴出来以供参考。
drivers/i2c/i2c-core-base.c中
/**
* i2c_transfer - execute a single or combined I2C message
* @adap: Handle to I2C bus
* @msgs: One or more messages to execute before STOP is issued to
* terminate the operation; each message begins with a START.
* @num: Number of messages to be executed.
*
* Returns negative errno, else the number of messages executed.
*
* Note that there is no requirement that each message be sent to
* the same slave address, although that is the most common model.
*/
int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msgs, int num)
{
int ret;
/* REVISIT the fault reporting model here is weak:
*
* - When we get an error after receiving N bytes from a slave,
* there is no way to report "N".
*
* - When we get a NAK after transmitting N bytes to a slave,
* there is no way to report "N" ... or to let the master
* continue executing the rest of this combined message, if
* that's the appropriate response.
*
* - When for example "num" is two and we successfully complete
* the first message but get an error part way through the
* second, it's unclear whether that should be reported as
* one (discarding status on the second message) or errno
* (discarding status on the first one).
*/
if (adap->algo->master_xfer) {
#ifdef DEBUG
for (ret = 0; ret < num; ret++) {
dev_dbg(&adap->dev,
"master_xfer[%d] %c, addr=0x%02x, len=%d%s\n",
ret, (msgs[ret].flags & I2C_M_RD) ? 'R' : 'W',
msgs[ret].addr, msgs[ret].len,
(msgs[ret].flags & I2C_M_RECV_LEN) ? "+" : "");
}
#endif
if (in_atomic() || irqs_disabled()) {
ret = i2c_trylock_bus(adap, I2C_LOCK_SEGMENT);
if (!ret)
/* I2C activity is ongoing. */
return -EAGAIN;
} else {
i2c_lock_bus(adap, I2C_LOCK_SEGMENT);
}
ret = __i2c_transfer(adap, msgs, num);
i2c_unlock_bus(adap, I2C_LOCK_SEGMENT);
return ret;
} else {
dev_dbg(&adap->dev, "I2C level transfers not supported\n");
return -EOPNOTSUPP;
}
}
EXPORT_SYMBOL(i2c_transfer);同文件中
/**
* __i2c_transfer - unlocked flavor of i2c_transfer
* @adap: Handle to I2C bus
* @msgs: One or more messages to execute before STOP is issued to
* terminate the operation; each message begins with a START.
* @num: Number of messages to be executed.
*
* Returns negative errno, else the number of messages executed.
*
* Adapter lock must be held when calling this function. No debug logging
* takes place. adap->algo->master_xfer existence isn't checked.
*/
int __i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msgs, int num)
{
unsigned long orig_jiffies;
int ret, try;
if (WARN_ON(!msgs || num < 1))
return -EINVAL;
if (adap->quirks && i2c_check_for_quirks(adap, msgs, num))
return -EOPNOTSUPP;
/*
* i2c_trace_msg_key gets enabled when tracepoint i2c_transfer gets
* enabled. This is an efficient way of keeping the for-loop from
* being executed when not needed.
*/
if (static_branch_unlikely(&i2c_trace_msg_key)) {
int i;
for (i = 0; i < num; i++)
if (msgs[i].flags & I2C_M_RD)
trace_i2c_read(adap, &msgs[i], i);
else
trace_i2c_write(adap, &msgs[i], i);
}
/* Retry automatically on arbitration loss */
orig_jiffies = jiffies;
for (ret = 0, try = 0; try <= adap->retries; try++) {
ret = adap->algo->master_xfer(adap, msgs, num);
if (ret != -EAGAIN)
break;
if (time_after(jiffies, orig_jiffies + adap->timeout))
break;
}
if (static_branch_unlikely(&i2c_trace_msg_key)) {
int i;
for (i = 0; i < ret; i++)
if (msgs[i].flags & I2C_M_RD)
trace_i2c_reply(adap, &msgs[i], i);
trace_i2c_result(adap, num, ret);
}
return ret;
}
EXPORT_SYMBOL(__i2c_transfer);
__i2c_transfer函数的核心是adap->algo->master_xfer(adap, msgs, num);这显然是之前已经建立好的函数指针指向的函数，也叫钩子函数。此时的master_xfer指向了谁呢？这个还真不好说，因为不同的显卡对应的adapter也不一样，比如amdgpu就与rk3399的不相同，机制也不完全一样。应该这样说，谁调用了drm_get_edid函数，谁在drm_get_edid函数的参数中提供了struct i2c_adapter *adapter，比如：
drivers/gpu/drm/radeon/radeon_connectors.c中的
radeon_connector->edid = drm_get_edid(connector, &radeon_connector->ddc_bus->aux.ddc);
drivers/gpu/drm/amd/amdgpu/amdgpu_connectors.c中的
amdgpu_connector->edid = drm_get_edid(connector, &amdgpu_connector->ddc_bus->aux.ddc);
drivers/gpu/drm/nouveau/nouveau_connector.c中的
nv_connector->edid = drm_get_edid(connector, i2c);
等。
因此在这里就不深入探究了，只看宏观功能，就是进行了i2c通信。
翻回头来看一下struct edid结构体的定义，include/drm/drm_edid.h中，定义如下：
struct edid {
u8 header[8];
/* Vendor & product info */
u8 mfg_id[2];
u8 prod_code[2];
u32 serial; /* FIXME: byte order */
u8 mfg_week;
u8 mfg_year;
/* EDID version */
u8 version;
u8 revision;
/* Display info: */
u8 input;
u8 width_cm;
u8 height_cm;
u8 gamma;
u8 features;
/* Color characteristics */
u8 red_green_lo;
u8 black_white_lo;
u8 red_x;
u8 red_y;
u8 green_x;
u8 green_y;
u8 blue_x;
u8 blue_y;
u8 white_x;
u8 white_y;
/* Est. timings and mfg rsvd timings*/
struct est_timings established_timings;
/* Standard timings 1-8*/
struct std_timing standard_timings[8];
/* Detailing timings 1-4 */
struct detailed_timing detailed_timings[4];
/* Number of 128 byte ext. blocks */
u8 extensions;
/* Checksum */
u8 checksum;
} __attribute__((packed));至此，Linux通过DRM获取EDID的流程就介绍完了。当然这只是粗线条的，更细节、更深入的内容还有待于进一步研究。
参考资料：
Linux DRM那些事-HDMI接口EDID获取（https://my.oschina.net/u/4702401/blog/5049682）