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

[Heiner Kallweit]

Am 07.06.2016 um 01:07 schrieb Mark Brown:
> On Mon, Jun 06, 2016 at 08:53:10PM +0200, Heiner Kallweit wrote:
>> Am 06.06.2016 um 19:40 schrieb Mark Brown:
> 
>>> That doesn't make any sense, the controller hardware doesn't 
>>> magically change based on what is connected to it.
> 
>> The issue with fsl-espi is that the controller deactivates CS after
>> each physical transfer. And a lot of HW designs use the hardware CS,
>> therefore the advise to use a GPIO instead doesn't really help.
> 
> There's no pinmux to fix the broken behaviour?  The Freescale SPI
> controllers really are terrible :(
> 
>> In case of SPI NOR CS needs to stay active between command and data
> 
> In the case of *all* SPI transactions this needs to happen.  This is
> *not* optional, it's not some weird requirement of NOR, it's one of the
> most basic requirements for a SPI driver on Linux.
> 
>> Well, we could reduce the max_transfer_size exposed by the driver by
>> the max length of a m25p80 read command in general.
> 
> No!  Apart from anything else this would be a complete and total
> abstraction failure.  This is a driver for a SPI controller, not a
> driver for a flash controller, which means that it should not have flash
> specific hacks in it.
> 
>> Then we might be too strict for other SPI devices, but this may
>> be acceptable as the current fsl-espi driver is usable with SPI NOR
>> devices only anyway.
> 
> What makes you say this?  I'm guessing that the driver is just broken
> but it would be good to understand the specifics.
> 
See fsl_espi_rw_trans():
In case of transfers > SPCOM_TRANLEN_MAX it loops and reads in
SPCOM_TRANLEN_MAX chunks whilst replacing bytes 2-4 of the
message buffer with the new start address to read from.
Having said that it implicitely assumes that the first transfer in
the message is a m25p80 read command with a 3 byte address.

And in fsl_espi_do_one_msg() it assumes that each message includes
max one r/o transfer (else rx_buf is overwritten).

I'm not 100% sure whether any arbitrary combination of r/o, r/w, w/o
transfers in a SPI message is acceptable. But this controller driver
supports just the transfer combinations generated by the m25p80
protocol driver.