转:让MT7620完美支持32M SPI Flash(W25Q256) — 兼谈设备驱动中的shutdown方法
http://blog.****.net/manfeel/article/details/43530817
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.