MT7620A 完美支持32M SPI Flash

前言

基于在Openwrt19.07开源软件中修改。初步测试Openwrt21.02无此bug.

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.

知识背景

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.

解决办法

转到drivers/mtd/devices/m25p80.c，所在目录为./build_dir/target-mipsel_24kec+dsp_uClibc-0.9.33.2/linux-ramips_mt7620a/linux-3.10.49/drivers/mtd/devices/m25p80.c

static void m25p_shutdown(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. */
    mtd_device_unregister(&flash->mtd);   
}


static struct spi_driver m25p80_driver = {
    .driver = {
        .name    = "m25p80",
        .owner    = THIS_MODULE,   //此处是3.14.28版本中的样式,4.4.14版本内容已改
    },
    .id_table    = m25p_ids,
    .probe    = m25p_probe,
    .remove    = m25p_remove,
    
    // manfeel, add shutdown method to reset spi flash
    .shutdown = m25p_shutdown,

    /* 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...
     */
};

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.

参考连接：https://blog.csdn.net/manfeel/article/details/43530817