[PATCH, 1/2] mtd: m25p80: Let m25p80_read() fallback to spi transfer

[Kamal Dasu]

On Wed, Jan 25, 2017 at 11:39 AM, Marek Vasut <marex at denx.de> wrote:
> On 01/25/2017 05:28 PM, Kamal Dasu wrote:
>> If the transfers are short and dest buffer or the flash address are
>> unaligned.
>
> That sounds like a DMA problem where you're trying to fall back to PIO ?
>
>> Also in case of older version of the controller there are
>> some address mapping limitations when a transfer crosses 4MB window
>> (addr + len).  So in such cases  need to fallback to normal MSPI
>> reads.
>
> But the driver can also detect this mode of failure before doing the
> transfer and call it's internal functions to perform the transfer as
> needed, right?
>
>> One other option is that controller divers implementation of
>> bcm_qspi_spi_flash_read() can return msg.retlen = 0 and the
>> m25p80_read() can fallback to normal mspi read.
>
> I'd much rather see the driver handling such detail internally instead
> of patching the core code. Moreover, if you patch the core code, the SF
> read will go - in case of a failure- all the way through the SPI
> framework only to land in the same driver, which doesn't make much sense.

Yes this is how the code was  organized before when I was making
initial commits. However I had to change it so that spi_flash_read()
can be exploited based on the review comments.  I was  handling code
internally using spi generic msg handling code for mspi transfer
fallback, but I was told that this code did not belong in the
controller driver. Hence the current implementation is geared towards
using spi transfer_one() in case of mspi transfers, without bothering
with how the messages are formed and pumped by the spi layer. If I
reorganize I am back to where I was before. Yes the code lands in the
same driver but it returns back to the m25p80 and uses the  normal
mspi reads.

> btw please do NOT top-post:
> http://www.arm.linux.org.uk/mailinglists/etiquette.php#e3
>

Sorry about that, I will make sure from this point forward I do not top-post.

>> Kamal
>>
>>
>>
>> On Tue, Jan 24, 2017 at 9:08 PM, Marek Vasut <marex at denx.de> wrote:
>>> On 01/24/2017 12:41 AM, Kamal Dasu wrote:
>>>> "ret can never be > 0 , it is only 0 or negative "
>>>>
>>>> I can fix this.
>>>>
>>>>>>> This looks really fragile and special-casing EINVAL here doesn't scale.
>>>>>>> But still, if your controller driver is buggy, fix the driver, do not
>>>>>>> pollute core code with workarounds. If you do support this sort of
>>>>>>> accelerated read and it fails, it means something is seriously wrong.
>>>>>>> If you need to invoke regular SPI reads to complete under some obscure
>>>>>>> circumstances, do it from the driver, not here.
>>>>>>
>>>>>> I guess the other half of m25p80_read can be factored out and used as
>>>>>> fallback from either m25p80_read or the controller driver.
>>>>>
>>>>> I think I see what you mean, but care to show an RFC patch ?
>>>>>
>>>>> --
>>>>
>>>> Its not the controller driver, but he hardware limitation with older
>>>> controller version. I have tried to see how I can do this better,
>>>> however when spi_flash_read() is called  cannot handle it within my
>>>> driver without returning from the function. I went over this with Mark
>>>> previously and this current solution seemed reasonable. Any other
>>>> solution outside of the generic driver would replicate a lot of code
>>>> unnecessarily.
>>>
>>> Hmmm, I kinda see the problem. I was thinking splitting the m25p80_read
>>> function could be the solution and invoking the second part from the
>>> driver if applicable, but this cannot work because the driver does not
>>> know when it's interacting with SPI NOR and when with something else .
>>>
>>> Can you tell me about the conditions under which the bcm controller
>>> fails and should fall back to standard spi read ?
>>>
>>> --
>>> Best regards,
>>> Marek Vasut
>
>
> --
> Best regards,
> Marek Vasut