Re: [PATCH 1/4] mmc: mmci: Bugfix in pio read for small packets

[Stefan Nilsson XK <stefan.xk.nilsson@stericsson.com>]

Hi Russel,

On 10/01/2011 06:09 PM, Russell King - ARM Linux wrote:
> Does this even work?  From the MMCI spec, I see no way for the MMCI
> peripheral to know the size of the read/write on the APB bus.
>
> The APB bus signals the MMCI uses are:
>
> PCLK - APB bus clock
> PRESETn - APB bus reset
> PADDR[11:2] - APB bus address
> PSEL - APB bus peripheral select
> PENABLE - APB bus enable
> PWRITE - APB bus write signal
> PWDATA[31:0] - APB bus write data
> PRDATA[31:0] - APB bus read data
>
> As you can see, nothing in that set indicates whether it's an 8-bit,
> 16-bit or 32-bit access.

> Moreover, if you read the MMCIFifoCnt register writeup:
>
>    The MMCIFifoCnt register contains the remaining number of words to be
>    written to or read from the FIFO. The FIFO counter loads the value
>    from the data length register (see Data length register, MMCIDataLength
>    on page 3-11) when the Enable bit is set in the data control register.
>    If the data length is not word aligned (multiple of 4), the remaining
>    1 to 3 bytes are regarded as a word.
>
> This suggests that we should be reading a 32-bit word and then storing
> the relevant bytes from it.

Hmm, that is true. I will try to rework the patch to skip the case and 
only do 32 bit reads. We do however need a patch since the current 
"count" calculation is wrong (more info below). But it does in fact work.

> The other thing which concerns me is that the MMCI (ARM Ltd one at least)
> only supports power-of-two block sizes.  So requesting a transfer of a
> single block with a block size of 3 bytes is not supported by the ARM Ltd
> MMCI.  (The way you end up with 1 to 3 bytes being received with ARM's
> MMCI is if you're using a streaming transfer.)

This is true for the standard ARM block. We have however done some 
modifications of the standard block and added support for SDIO which 
also implies streaming transfers which can be non power of 2. So with 
the current implementation:

	readsl(base + MMCIFIFO, ptr, count >> 2);

when we get a request to read 3, 2 or 1 bytes (which our WLAN driver 
actually requests from time to time), this will result in reading 0 
words which is not correct. This was the original problem, but I might 
have "overdone" the solution a bit. I will upload a new patch which 
solves the original problem in a better way.

> The last thing I don't like about this patch is that this code is in a
> really hot path - one which is absolutely critical for things to work -
> and the need for the condition to be dealt with is only at the end of a
> transfer, not each time the FIFO needs emptying.
>
> Bear in mind that there are platforms with the ARM MMCI which must read
> the data within a certain time or suffer overruns, and which have either
> totally broken and useless DMA or no DMA capability at all (which are
> the only platforms I have with a MMCI on.)

I will keep this in mind. We are fortunately blessed with a working DMA 
in our platform, but for small SDIO transfers there is no point in 
setting up a DMA job which is when we fall back to PIO mode.

Best Regards

Stefan Nilsson