On 02/01/16 14:29, Russell King - ARM Linux wrote:
> On Tue, Dec 29, 2015 at 03:44:38PM +0200, Adrian Hunter wrote:
>> On 21/12/15 13:41, Russell King wrote:
>>> Unnecessarily mapping and unmapping the align buffer for SD cards is
>>> expensive: performance measurements on iMX6 show that this gives a hit
>>> of 10% on hdparm buffered disk reads.
>>>
>>> MMC/SD card IO comes from the mm/vfs which gives us page based IO, so
>>> for this case, the align buffer is not going to be used. However, we
>>> still map and unmap this buffer.
>>>
>>> Eliminate this by switching the align buffer to be a DMA coherent
>>> buffer, which needs no DMA maintenance to access the buffer.
>>
>> Did you consider putting the align buffer in the same allocation
>> as the adma_table?
>
> It's not clear whether host->adma_table_sz would be appropriately
> aligned.
>
>>> @@ -3003,6 +2972,10 @@ int sdhci_add_host(struct sdhci_host *host)
>>> host->adma_table_sz,
>>> &host->adma_addr,
>>> GFP_KERNEL);
>>> + host->align_buffer = dma_alloc_coherent(mmc_dev(mmc),
>>> + host->align_buffer_sz,
>>> + &host->align_addr,
>>> + GFP_KERNEL);
>>> host->align_buffer = kmalloc(host->align_buffer_sz, GFP_KERNEL);
>>
>> kmalloc line is still there
>
> Good catch, thanks.
>
>>> +
>>> + /* dma_alloc_coherent returns page aligned and sized buffers */
>>> + BUG_ON(host->align_addr & host->align_mask);
>>
>> It would be nicer not to have any BUG_ON()
>
> This is a situation that should _never_ occur (if it does, then the
> dma_alloc_coherent() implementation is violating the API requirements,
> which are to return a page-sized page-aligned buffer.) I guess it
> could be a WARN_ON(), but if it fails we're likely to cause data
> corruption. So, a WARN_ON() and failing the probe seems more
> appropriate - but then that means yet more messy cleanup in an already
> messy part of the driver.
Isn't there already a cleanup path? i.e.
} else if (host->adma_addr & host->align_mask) {
pr_warn("%s: unable to allocate aligned ADMA descriptor\n",
mmc_hostname(mmc));
host->flags &= ~SDHCI_USE_ADMA;
dma_free_coherent(mmc_dev(mmc), host->adma_table_sz,
host->adma_table, host->adma_addr);
kfree(host->align_buffer);
host->adma_table = NULL;
host->align_buffer = NULL;
}
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