[PATCH 2/2] mtd: m25p80: consider max_transfer_size when reading

[Heiner Kallweit]

Am 06.05.2016 um 14:14 schrieb Mark Brown:
> On Thu, May 05, 2016 at 04:57:00PM -0700, Brian Norris wrote:
>> On Thu, Apr 07, 2016 at 09:09:56PM +0200, Heiner Kallweit wrote:
> 
>>> 2. At least in the case of fsl-espi the size limit refers to one
>>>    physical transfer (including the command) and therefore to the sum
>>>    of all transfers.
>>> We should change
>>> +	t[1].len = min_t(size_t, len, spi_max_transfer_size(spi));
>>> to
>>> +	t[1].len = min(len, spi_max_transfer_size(spi) - t[0].len);
>>>
>>> Apart from that the patch set looks good to me.
> 
>> That's not what Mark specified here:
> 
>> http://lists.infradead.org/pipermail/linux-mtd/2015-November/063616.html
> 
>> and that's not what the API's very *name* means; it says max transfer
>> size (where a spi_transfer is a very well-defined concept). You need to
>> fix the driver or take up the API issues with Mark if you want to
>> suggest we interpret this differently.
> 
> Yes, it's called the maximum transfer size because it is the maximum
> size of a transfer, not because it's the maximum size of a message.
> 
I'd like to come back to this discussion. You said best would be to fix
the chip driver. To do this and calculate an appropriate value for
max_transfer_size the chip driver would have to know that the spi_device
is a spi-nor device.
We could check that the name of the driver bound to the spi-device is m25p80.
This doesn't sound very nice.

Or maybe better, struct spi_device could be extended by a pointer to a struct spi_nor.
This comment in the definition of struct spi_device seems to point in this direction:
"likely need more hooks for more protocol options affecting how the controller
talks to each chip"
Just storing the length of the read command + dummy bytes would also be sufficient.
Could this be a feasible way?