Re: [PATCH] mmc: core: don't decrement qty when calculating max_discard

[Stephen Warren <swarren-3lzwWm7+Weoh9ZMKESR00Q@public.gmane.org>]

On 12/18/2013 04:08 AM, Adrian Hunter wrote:
> On 17/12/13 20:02, Stephen Warren wrote:
>> From: Stephen Warren <swarren-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
>>
>> In mmc_do_calc_max_discard(), if any value has been assigned to qty,
>> that value must have passed the timeout checks in the loop. Hence,
>> qty is the maximum number of erase blocks that fit within the timeout,
>> not the first value that does not fit into the timeout. In turn, this
>> means we don't need any special case for (qty == 1); any value of qty
>> needs to be multiplied by the card's erase shift, and we don't need to
>> decrement qty before doing so.
>>
>> Without this patch, on the NVIDIA Tegra Cardhu board, the loops result
>> in qty == 1, which is immediately returned. This causes discard to
>> operate a single sector at a time, which is chronically slow. With this
>> patch in place, discard operates a single erase block at a time, which
>> is reasonably fast.
...
> The quantity is decreased by 1 to account for the fact that the erase can
> cross the boundary between 1 erase block and another. i.e. even though the
> size is 1 erase block it touches 2 erase blocks.

Don't erases have to be aligned to the erase block alignment; surely
that's what the eMMC's preferred erase alignment is all about?

If that isn't the case, a comment in the code would be extremely useful...