Re: Dynamic MMC device naming vs. bootloaders

[Chris Ball <cjb@laptop.org>]

Hi Stephen,

On Mon, Apr 04 2011, Stephen Warren wrote:
> Chris et. al.,
>
> I'm working on an ARM system that contains at least two MMC/SD devices;
> specifically the board has an internal MMC device, and an SD card slot,
> although the SoC has four MMC/SD hosts IIRC.
>
> The kernel's naming of these devices is dynamic. If the SD card is not
> plugged in, the internal MMC is always known as mmcblock0. If the SD card
> is plugged in too, sometimes the internal MMC is mmcblk0 and sometimes it's
> mmcblk1. I assume this is timing related; 2.6.37 usually seemed to name
> them "in order", whereas 2.6.38 usually seems to name them "backwards".

The standard way to solve this is to use an initrd that performs the
naming (and mounting) that you'd like based on characteristics of the
device or card -- this could include making mmcblk{0,1} always attach
to the controllers that you're expecting it to.  It's hard enough to get
device naming policy right that we don't usually try to do it in-kernel.

> This causes problems with the bootloader scripts I'm using, which assumes
> that the internal MMC is always device 0 and the SD slot is always device 1,
> and hence provides kernel command-line argument root=/dev/mmcblk0p3 or
> root=/dev/mmcblk1p3 based on whether it booted from SD or internal MMC (SD
> is searched for a valid image first by the bootloader).
>
> This could be solved by naming the kernel MMC devices statically based on
> the host controller, rather than dynamically based on the first available
> ID when the actual storage is detected. The patch below implements this.
>
> Is this patch conceptually acceptable for the MMC driver? Obviously, a
> complete patch would need to also remove the dev_use structure etc.
>
> Another solution might be for the kernel command-line to specify the
> filesystem UUID, rather than a device node. However, this entails much more
> work for the bootloader. I'm not sure whether U-Boot can do this right now?
> And actually, the filesystem images I'm using don't always have a UUID by
> dDefault IIRC.
>
> diff --git a/drivers/mmc/card/block.c b/drivers/mmc/card/block.c
> index bfc8a8a..5131b02 100644
> --- a/drivers/mmc/card/block.c
> +++ b/drivers/mmc/card/block.c
> @@ -581,7 +581,7 @@ static struct mmc_blk_data *mmc_blk_alloc(struct mmc_card *card)
>         struct mmc_blk_data *md;
>         int devidx, ret;
>  
> -       devidx = find_first_zero_bit(dev_use, max_devices);
> +       devidx = card->host->index;
>         if (devidx >= max_devices)
>                 return ERR_PTR(-ENOSPC);
>         __set_bit(devidx, dev_use);

Hm, will think about this, thanks.

- Chris.
-- 
Chris Ball   <cjb@laptop.org>   <http://printf.net/>
One Laptop Per Child