[U-Boot] [PATCH 2/2] mmc: tegra: invalidate complete cachelines

[Simon Glass]

Hi Stephen,

On Fri, Nov 2, 2012 at 1:22 PM, Stephen Warren <swarren at wwwdotorg.org> wrote:
> On 04/26/2012 11:29 PM, Mike Frysinger wrote:
>> On Thursday 26 April 2012 06:34:43 Thierry Reding wrote:
>>> For reference, see sd_change_freq() in drivers/mmc/mmc.c.
>
> This is a follow-up to:
>
> http://lists.denx.de/pipermail/u-boot/2012-April/123080.html
>
> which was referenced from:
>
> http://lists.denx.de/pipermail/u-boot/2012-September/133641.html
>
>> yes, that shows what we're talking about. int sd_change_freq(struct
>> mmc *mmc) { ... stack vars ... int err;
>> ALLOC_CACHE_ALIGN_BUFFER(uint, scr, 2); struct mmc_data data; int
>> timeout; ... stack vars ... ... data.dest = (char *)scr;
>> data.blocksize = 8; data.blocks = 1; data.flags = MMC_DATA_READ;
>> err = mmc_send_cmd(mmc, &cmd, &data); ...
>>
>> static int mmc_send_cmd(struct mmc *mmc, struct mmc_cmd *cmd,
>> struct mmc_data *data) { ... if (data->flags & MMC_DATA_READ) { if
>> ((uintptr_t)data->dest & (ARCH_DMA_MINALIGN - 1)) printf("Warning:
>> unaligned read from %p " "may fail\n", data->dest);
>> invalidate_dcache_range((ulong)data->dest, (ulong)data->dest +
>> data->blocks * data->blocksize); } ...
>>
>> so what invalidate_dcache_range() will invalidate is from &scr and
>> 8 bytes after that.  the higher layers make sure &scr is aligned
>> properly, but not that it spans a full cache line.  so if the stack
>> layout is: [random stuff] 0x100       [scr[0]] 0x104  [scr[1]] 0x108
>> [err] 0x10a   [timeout] [more stuff]
>
> That's not the stack layout. scr is allocated (as quoted above) using
> ALLOC_CACHE_ALIGN_BUFFER. Therefore, the storage space allocated for
> scr is always cache-aligned in address and size, even if the code only
> uses 8 bytes of it.
>
>> this implicitly invalidates [err] and [timeout] and more stuff.  by
>> you forcibly rounding up the length, you no longer get a warning,
>> but you still (silently) blew away those variables from cache
>> possibly losing changes that weren't written back to external
>> memory yet.
>
> So, this problem won't actually occur here.
>
> So, Thierry's proposed solution (which I'll re-quote below) would
> actually work just fine:
>
>> [PATCH 2/2] mmc: tegra: invalidate complete cachelines
>>
>> The MMC core sometimes reads buffers that are smaller than a
>> complete cacheline, for example when reading the SCR. In order to
>> avoid a warning from the ARM v7 cache handling code, this patch
>> makes sure that complete cachelines are flushed.
>>
>> Signed-off-by: Thierry Reding <thierry.reding at
>> avionic-design.de> --- drivers/mmc/tegra2_mmc.c |    7 ++++--- 1
>> file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/mmc/tegra2_mmc.c b/drivers/mmc/tegra2_mmc.c
>> index fb8a57d..3615876 100644 --- a/drivers/mmc/tegra2_mmc.c +++
>> b/drivers/mmc/tegra2_mmc.c @@ -323,12 +323,13 @@ static int
>> mmc_send_cmd(struct mmc *mmc, struct mmc_cmd *cmd, } writel(mask,
>> &host->reg->norintsts); if (data->flags & MMC_DATA_READ) { +
>> ulong end = (ulong)data->dest + +                             roundup(data->blocks *
>> data->blocksize, +                                            ARCH_DMA_MINALIGN); if
>> ((uintptr_t)data->dest & (ARCH_DMA_MINALIGN - 1)) printf("Warning:
>> unaligned read from %p " "may fail\n", data->dest); -
>> invalidate_dcache_range((ulong)data->dest, -                          (ulong)data->dest
>> + -                                   data->blocks * data->blocksize); +
>> invalidate_dcache_range((ulong)data->dest, end); } }
>>
>> --
>
> ... so long as we require any struct mmc_data data's .dest field to
> point at a buffer that was allocated using ALLOC_CACHE_ALIGN_BUFFER.
>
> So, I'd like to propose that we take Thierry's fix, perhaps having
> validated (or implemented some way of enforcing) that
> ALLOC_CACHE_ALIGN_BUFFER() was used to allocate the buffer pointer.

That sounds ok to me.

It is important to avoid silent failure if we can. Are you proposing
to pass a 'size' parameter along with any buffer pointer, or something
else?

Regards,
Simon