Re: [PATCHv2 4/7] input/cma3000_d0x: Add CMA3000 spi support

[Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>]

Hello Jonathan

 I just send the mail with the latest patch version. I will wait for
hermanthv to see if he wants to replace the msg from the read/write
functions, and I will follow his style for the cmr3000. I have testest
the latest changes on my board and it looks ok.

  Thanks for your feedback, and I hope it is ok now :)

On Tue, Oct 18, 2011 at 17:27, Jonathan Cameron <jic23@cam.ac.uk> wrote:
> On 10/18/11 16:18, Ricardo Ribalda Delgado wrote:
>> Hello Jonathan
>>
>>  I have separated the read and write commands... but when I was
>> implementing the spi_write I found that most of the drivers just use
>> stack variables for spi_write. Like drivers/hwmon/ads7871.c,
>> drivers/gpio/max7301.c and others... Am I missing something or it is a
>> general "bug"?
> bug unless I am also missing where they prevent dma
> transfers.  spi_read_then_write is fine
> as it does a copy, but spi_write doesn't.
> Documentation/spi/spi-summary
>
> "Note that there are two types of memory your driver must manage as part
> of interacting with SPI devices.
>
>  - I/O buffers use the usual Linux rules, and must be DMA-safe.
>    You'd normally allocate them from the heap or free page pool.
>    Don't use the stack, or anything that's declared "static".
> "
>
> This was an issue many people (including me) weren't aware of until
> a year or two ago when it was picked up in reviews of a number of drivers.
> It was around that time the ___cacheline_aligned trick was used by
> Michael Hennerich and everyone else picked up on that as often the
> easiest way of doing this in a driver.
>
> Don't suppose there is anything stopping you doing
> spi_write_then_read(tx, 2, NULL, 0)
> and using the buffers helpfully allocated in the spi core?
> This'll allocate extra space if someone else is using the core
> bounce buffers though...
>
>>
>> As you say, it is better to not rewrite the cma3000 driver until
>> Hemanth says so, lets leave it for a future patch.
>>
>>
>>
>>
>> On Tue, Oct 18, 2011 at 15:50, Jonathan Cameron <jic23@cam.ac.uk> wrote:
>>> On 10/18/11 14:43, Ricardo Ribalda Delgado wrote:
>>>> Hello Jonathan
>>>>
>>>>   First of all, thanks for your messages :).
>>>>
>>>>> To make my point about these functions being more complex than needed
>>>>> in more detail....
>>>>>
>>>>> If this were two functions and you drop the zero and 1 mask
>>>>> (which I'm not convinced make any sense. I've also killed the message.
>>>>> We both agree it is the wrong way to go, so post a patch fixing the i2c
>>>>> interface as well.
>>>>
>>>> Of course your functions are much more simpler and beautiful than the
>>>> fat one I wrote, no doubt about it :). Just three comments
>>>>
>>>> - Checking the one mask and the zero mask is the only way we have to
>>>> know if the chip is still there, The absense of that reply should
>>>> trigger an IO error or at least a retry. As you point out, the
>>>> zero/one mask is only violated on startup.  I just wanted to make it
>>>> more risk free, but if you believe it is more clear that way, lets
>>>> remove it
>>> It's somewhat unconventional to verify the existence of a chip like this.
>>> Usually you assume that if it was there once it still is unless there
>>> is a very good reason to think otherwise.  Worth doing an initial check
>>> in your spi_probe and indeed verify there against these known bits.
>>> No need to do it every time though.
>>>>
>>>> - I am not very fun of kmallocing data per write, specially when it is
>>>> part of the irq handler, and you expect this to be low latency. What
>>>> about allocating a buffer on init time, and use it with a mutex?
>>> That's absolutely fine and the right way to do it. You could poke it
>>> into the cma3000_accl_data then use the cachline aligned magic. Its
>>> is tiny so I doubt anyone will mind the overhead for the i2c side of
>>> things.
>>>>
>>>> -I dont like the push error message to the bottom, but that will mean
>>>> a rewrite of the cma3000 driver, shall I go for it?
>>> I would. Though probably worth getting Hemanth to say if he minds first
>>> given it's his driver!
>>>>
>>>>
>>>> Thanks again, and  I will post the new version when you reply this :)
>>>>
>>>>
>>>
>>>
>>
>>
>>
>
>



-- 
Ricardo Ribalda
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