让MT7620完美支持32M SPI Flash(W25Q256) — 兼谈设备驱动中的shutdown方法

前言

OpenWrt的最新kernel(3.14.28)已经能够支持32M SPI Flash的读写以及擦除操作.然而,可能是系统考虑不周,亦或是MT7620系统的BUG,在配置了W25Q256的MT7620开发板系统上,无法soft reset!经过查阅相关资料,发现,MT7620默认支持24bit(3byte)的spi地址模式,而要支持32M以上的spi flash,则必须切换到32bit(4byte)地址模式.在soft reset的时候,spi停留在了32bit模式,没有切换回默认的24bit模式,导致reset后,MT7620在默认的24bit模式,无法和32bit模式的spi通讯,系统死机.那么问题来了:如何在soft reset时刻,让spi flash切换回24bit模式呢?本文通过设备驱动中的一个shutdown方法来解决这个问题.

背景知识

在linux源代码kernel目录下,有一个reboot.c文件,里面暴露了一个register_reboot_notifier方法,可以让kernel中的代码有机会获得reboot的通知,当我们继续分析reboot.c的代码时,会发现更有意思的东西:

 /**
  *  kernel_restart - reboot the system
  *  @cmd: pointer to buffer containing command to execute for restart
  *      or %NULL
  *
  *  Shutdown everything and perform a clean reboot.
  *  This is not safe to call in interrupt context.
  */
 void kernel_restart(char *cmd)
 {
     kernel_restart_prepare(cmd);
     migrate_to_reboot_cpu();
     syscore_shutdown();
     if (!cmd)
         pr_emerg("Restarting system\n");
     else
         pr_emerg("Restarting system with command '%s'\n", cmd);
     kmsg_dump(KMSG_DUMP_RESTART);
     machine_restart(cmd);
 }

在kernel_restart中,又调用了kernel_restart_prepare方法:

 void kernel_restart_prepare(char *cmd)
 {
     blocking_notifier_call_chain(&reboot_notifier_list, SYS_RESTART, cmd);
     system_state = SYSTEM_RESTART;
     usermodehelper_disable();
     device_shutdown();
 }

device_shutdown在drivers/base/core.c中实现:

 /**
  * device_shutdown - call ->shutdown() on each device to shutdown.
  */
 void device_shutdown(void)
 {
     struct device *dev, *parent;

     spin_lock(&devices_kset->list_lock);
     /*
      * Walk the devices list backward, shutting down each in turn.
      * Beware that device unplug events may also start pulling
      * devices offline, even as the system is shutting down.
      */
     while (!list_empty(&devices_kset->list)) {
         dev = list_entry(devices_kset->list.prev, struct device,
                 kobj.entry);

         /*
          * hold reference count of device's parent to
          * prevent it from being freed because parent's
          * lock is to be held
          */
         parent = get_device(dev->parent);
         get_device(dev);
         /*
          * Make sure the device is off the kset list, in the
          * event that dev->*->shutdown() doesn't remove it.
          */
         list_del_init(&dev->kobj.entry);
         spin_unlock(&devices_kset->list_lock);

         /* hold lock to avoid race with probe/release */
         if (parent)
             device_lock(parent);
         device_lock(dev);

         /* Don't allow any more runtime suspends */
         pm_runtime_get_noresume(dev);
         pm_runtime_barrier(dev);

         // manfeel, add debug info
         //dev_info(dev,"search shutdown method...\n");

         if (dev->bus && dev->bus->shutdown) {
             //if (initcall_debug) manfeel
                 dev_info(dev, "shutdown\n");
             dev->bus->shutdown(dev);
         } else if (dev->driver && dev->driver->shutdown) {
             //if (initcall_debug) manfeel
                 dev_info(dev, "shutdown\n");
             dev->driver->shutdown(dev);
         }

         device_unlock(dev);
         if (parent)
             device_unlock(parent);

         put_device(dev);
         put_device(parent);

         spin_lock(&devices_kset->list_lock);
     }
     spin_unlock(&devices_kset->list_lock);
     async_synchronize_full();
 }

通过阅读代码,我们不难发现,在device_shutdown中,枚举了设备的shutdown方法,如果存在该方法,则会调用之.

于是,32M spi flash的reset方法喷薄而出.

解决办法

转到drivers/mtd/devices/m25p80.c

修改如下代码:

 static int m25p_remove(struct spi_device *spi)
 {
     struct m25p *flash = spi_get_drvdata(spi);

     // manfeel note: add spi flash reset code
     flash->command[0] = 0x66;
     spi_write(flash->spi, flash->command, 1);
     flash->command[0] = 0x99;
     spi_write(flash->spi, flash->command, 1);
     /* Clean up MTD stuff. */
     return mtd_device_unregister(&flash->mtd);
 }


 static struct spi_driver m25p80_driver = {
     .driver = {
         .name   = "m25p80",
         .owner  = THIS_MODULE,
     },
     .id_table   = m25p_ids,
     .probe  = m25p_probe,
     .remove = m25p_remove,
     // manfeel, add shutdown method to reset spi flash
     .shutdown = m25p_remove,

     /* REVISIT: many of these chips have deep power-down modes, which
      * should clearly be entered on suspend() to minimize power use.
      * And also when they're otherwise idle...
      */
 };

总结

通过注册设备的shutdown方法,让我们有机会在系统重启的时刻,做一些deinit的操作.通过此种方法来复位spi flash,优雅而简洁.