Re: [PATCH] mmc: mmci: Support non-power-of-two block sizes for ux500v2 variant

["Per Förlin" <per.forlin@stericsson.com>]

On 11/22/2012 06:37 PM, Russell King - ARM Linux wrote:
> On Thu, Nov 22, 2012 at 06:28:30PM +0100, Per Forlin wrote:
>> On Wed, Nov 21, 2012 at 5:50 PM, Russell King - ARM Linux
>> <linux@arm.linux.org.uk> wrote:
>>> On Wed, Nov 21, 2012 at 05:13:55PM +0100, Per Forlin wrote:
>>>> On Wed, Nov 21, 2012 at 4:38 PM, Russell King - ARM Linux
>>>> <linux@arm.linux.org.uk> wrote:
>>>>> On Fri, Oct 12, 2012 at 04:02:02PM +0200, Ulf Hansson wrote:
>>>>>>  /*
>>>>>> + * Validate mmc prerequisites
>>>>>> + */
>>>>>> +static int mmci_validate_data(struct mmci_host *host,
>>>>>> +                           struct mmc_data *data)
>>>>>> +{
>>>>>> +     if (!data)
>>>>>> +             return 0;
>>>>>> +
>>>>>> +     if (!host->variant->non_power_of_2_blksize &&
>>>>>> +         !is_power_of_2(data->blksz)) {
>>>>>> +             dev_err(mmc_dev(host->mmc),
>>>>>> +                     "unsupported block size (%d bytes)\n", data->blksz);
>>>>>> +             return -EINVAL;
>>>>>> +     }
>>>>>> +
>>>>>> +     if (data->sg->offset & 3) {
>>>>>> +             dev_err(mmc_dev(host->mmc),
>>>>>> +                     "unsupported alginment (0x%x)\n", data->sg->offset);
>>>>>> +             return -EINVAL;
>>>>>> +     }
>>>>>
>>>>> Why?  What's the reasoning behind this suddenly introduced restriction?
>>>>> readsl()/writesl() copes just fine with non-aligned pointers.  It may be
>>>>> that your DMA engine can not, but that's no business interfering with
>>>>> non-DMA transfers, and no reason to fail such transfers.
>>>>>
>>>>> If your DMA engine can't do that then its your DMA engine code which
>>>>> should refuse to prepare the transfer.
>>>>>
>>>>> Yes, that means problems with the way things are ordered - or it needs a
>>>>> proper API where DMA engine can export these kinds of properties.
>>>> The alignment constraint is related to PIO, sg_miter and that FIFO
>>>> access must be done with 4 bytes.
>>>
>>> Total claptrap.  No it isn't.
>>>
>>> - sg_miter just deals with bytes, and number of bytes transferred; there
>>>   is no word assumptions in that code.  Indeed many ATA disks transfer
>>>   by half-word accesses so such a restriction would be insane.
>>>
>>> - the FIFO access itself needs to be 32-bit words, so readsl or writesl
>>>   (or their io* equivalents must be used).
>>>
>>> - but - and this is the killer item to your argument as I said above -
>>>   readsl and writesl _can_ take misaligned pointers and cope with them
>>>   fine.
>>>
>>> The actual alignment of the buffer address is totally irrelevant here.
>>>
>>> What isn't irrelevant is the _number_ of bytes to be transferred, but
>>> that's something totally different and completely unrelated from
>>> data->sg->offset.
>> Let's try again :)
>>
>> Keep in mind that the mmc -block layer is aligned so from that aspect
>> everything is fine.
>> SDIO may have any length or alignment but sg-len is always 1.
>>
>> There is just one sg-element and one continues buffer.
>>
>> sg-miter splits the continues buffer in chunks that may not be aligned
>> with 4 byte size. It depends on the start address alignment of the
>> buffer.
>>
>> Is it more clear now?
> 
> Is this more clear: you may be passed a single buffer which is misaligned.
> We cope with that just fine.  There is *no* reason to reject that transfer
> because readsl/writesl cope just fine with it.
> 
The MMCI driver doesn't support alignment smaller than 4 bytes (it may result in data corruption).
There are two options
1. Either one should fix the driver to support it. Currently the driver only supports miss-alignment of the last sg-miter buffer.
2. Or be kind to inform the user that the alignment is not supported.

BR
Per