Re: [PATCH v3 3/6] spi: sun6i: restrict transfer length in PIO-mode

From: Sergey Suloev <ssuloev@orpaltech.com>
To: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Chen-Yu Tsai <wens@csie.org>, Mark Brown <broonie@kernel.org>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-spi@vger.kernel.org
Subject: Re: [PATCH v3 3/6] spi: sun6i: restrict transfer length in PIO-mode
Date: Mon, 9 Apr 2018 14:59:57 +0300	[thread overview]
Message-ID: <0e2fefa5-b6e7-5e42-cf6e-8fc921f972dd@orpaltech.com> (raw)
In-Reply-To: <20180409113603.2iexkqvyeqmysp5e@flea>

On 04/09/2018 02:36 PM, Maxime Ripard wrote:
> On Mon, Apr 09, 2018 at 02:10:40PM +0300, Sergey Suloev wrote:
>> On 04/09/2018 01:50 PM, Mark Brown wrote:
>>> On Mon, Apr 09, 2018 at 01:26:23PM +0300, Sergey Suloev wrote:
>>>> On 04/09/2018 12:27 PM, Maxime Ripard wrote:
>>>>> On Fri, Apr 06, 2018 at 06:48:23PM +0300, Sergey Suloev wrote:
>>>>>> On 04/06/2018 10:34 AM, Maxime Ripard wrote:
>>>>>> According to what you said the driver must implement
>>>>>> "transfer_one_message" instead of "transfer_one"
>>>>> I'm not sure what makes you think that I said that.
>>>> Because current implementation tries to send more than FIFO-depth of data in
>>>> a single call to "transfer_one" which is wrong.
>>> No, that's absolutely not the case.  All any of these functions has to
>>> do is transfer whatever they were asked to, how they do it is not at all
>>> important to the framework.
>> I think you don't fully understand the issue. Let's talk about sun4i and
>> sun6i SPI  drivers separately.
>>
>> sun4i
>>
>> 1)it is correctly declaring max_transfer_size=FIFO depth for PIO mode  but
>> transfer_one() function doesn't follow the declaration allowing PIO
>> transfers longer than FIFO depth  by just refilling FIFO using 3/4 FIFO
>> empty interrupt. I can definitely state here that long transfers WON'T WORK
>> on real hardware.
> Surely the original author of the patch allowing to do just that
> disagrees with you.
I am not getting the point why the driver is declaring the max transfer 
length value and not following the rule.
> And it's not about the hardware itself, it's about
> how the driver operates as well.
>
>> I tested it and that's why I can say that.
> Then it must be fixed, and not silently reverted.
>
>> But as soon as sun4i SPI driver  is correctly declaring
>> max_transfer_size then "smart" clients will work well by limiting a
>> single transfer size to FIFO depth. I tested it with real hardware,
>> again.
> This is really not my point. What would prevent you from doing
> multiple transfers in that case, and filling the FIFO entirely,
> waiting for it to be done, then resuming until you have sent the right
> number of bytes?
Because it makes no sense IMHO. I can't see any single point in allowing 
long PIO transfers. Can you find at least one ?

I think we should reuse as much SPI core code as possible. The SPI core 
can handle an SPI message with multiple transfers,
all we need is to have max_transfer_size = FIFO depth and restrict it in 
transfer_one().

>
> Maxime
>
>
>
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