Re: [linux-sunxi] [PATCH v3 10/13] spi: sunxi: merge sun4i and sun6i SPI driver

[Julian Calaby <julian.calaby@gmail.com>]

Hi Michal,

On Tue, Jun 14, 2016 at 4:35 PM, Michal Suchanek <hramrach@gmail.com> wrote:
> Hello,
>
> On 14 June 2016 at 07:45, Julian Calaby <julian.calaby@gmail.com> wrote:
>> Hi Michal,
>>
>> On Tue, Jun 14, 2016 at 3:28 PM, Michal Suchanek <hramrach@gmail.com> wrote:
>>> On 14 June 2016 at 06:47, Julian Calaby <julian.calaby@gmail.com> wrote:
>>>> Hi Michal,
>>>>
>>>> On Tue, Jun 14, 2016 at 2:34 PM, Michal Suchanek <hramrach@gmail.com> wrote:
>>>>> Hello,
>>>>>
>>>>> On 14 June 2016 at 01:43, Julian Calaby <julian.calaby@gmail.com> wrote:
>>>>>> Hi Michal,
>>>>>>
>>>>>> On Tue, Jun 14, 2016 at 3:46 AM, Michal Suchanek <hramrach@gmail.com> wrote:
>>>>>>> The drivers are very similar and share multiple flaws which needed
>>>>>>> separate fixes for both drivers.
>>>>>>>
>>>>>>> Signed-off-by: Michal Suchanek <hramrach@gmail.com>
>>>>>>> ---
>>>>>>>  drivers/spi/Kconfig     |   8 +-
>>>>>>>  drivers/spi/Makefile    |   1 -
>>>>>>>  drivers/spi/spi-sun4i.c | 156 +++++++++++--
>>>>>>>  drivers/spi/spi-sun6i.c | 598 ------------------------------------------------
>>>>>>>  4 files changed, 143 insertions(+), 620 deletions(-)
>>>>>>>  delete mode 100644 drivers/spi/spi-sun6i.c
>>>>>>>
>>>>>>> diff --git a/drivers/spi/spi-sun4i.c b/drivers/spi/spi-sun4i.c
>>>>>>> index 0b8e6c6..c76f8e4 100644
>>>>>>> --- a/drivers/spi/spi-sun4i.c
>>>>>>> +++ b/drivers/spi/spi-sun4i.c
>>>>>>> @@ -279,9 +321,14 @@ static int sunxi_spi_transfer_one(struct spi_master *master,
>>>>>>>         reg = sunxi_spi_read(sspi, SUNXI_TFR_CTL_REG);
>>>>>>>
>>>>>>>         /* Reset FIFOs */
>>>>>>> -       sunxi_spi_write(sspi, SUNXI_TFR_CTL_REG,
>>>>>>> -                       reg | sspi_bits(sspi, SUNXI_CTL_RF_RST) |
>>>>>>> -                       sspi_bits(sspi, SUNXI_CTL_TF_RST));
>>>>>>> +       if (sspi->type == SPI_SUN4I)
>>>>>>> +               sunxi_spi_write(sspi, SUNXI_TFR_CTL_REG,
>>>>>>> +                               reg | sspi_bits(sspi, SUNXI_CTL_RF_RST) |
>>>>>>> +                               sspi_bits(sspi, SUNXI_CTL_TF_RST));
>>>>>>> +       else
>>>>>>> +               sunxi_spi_write(sspi, SUNXI_FIFO_CTL_REG,
>>>>>>> +                               sspi_bits(sspi, SUNXI_CTL_RF_RST) |
>>>>>>> +                               sspi_bits(sspi, SUNXI_CTL_TF_RST));
>>>>>>
>>>>>> If we're already doing different stuff for each generation of the IP,
>>>>>> why not just use the register offsets and bit definitions directly?
>>>>>
>>>>> Because having (*sspi->regmap)[SUNXI_FIFO_CTL_REG] all over the place
>>>>> makes my eyes bleed and you cannot use the check that you are
>>>>> accessing a register that actually exists.
>>>>
>>>> I mean removing SUNXI_CTL_RF_RST and SUNXI_CTL_TF_RST from all of the
>>>> indirection you added and using them directly, i.e.
>>>>
>>>> #define SUN4I_TFR_CTL_RF_RST BIT(x)
>>>> #define SUN4I_TFR_CTL_TF_RST BIT(x)
>>>> #define SUN6I_FIFO_CTL_RF_RST BIT(x)
>>>> #define SUN6I_FIFO_CTL_TF_RST BIT(x)
>>>>
>>>> then
>>>>
>>>> if (sspi->type == SPI_SUN4I)
>>>>     sunxi_spi_write(sspi, SUNXI_TFR_CTL_REG, reg |
>>>> SUN4I_TFR_CTL_RF_RST | SUN4I_TFR_CTL_TF_RST);
>>>> else
>>>>     sunxi_spi_write(sspi, SUNXI_FIFO_CTL_REG, reg |
>>>> SUN6I_FIFO_CTL_RF_RST | SUN6I_FIFO_CTL_TF_RST);
>>>>
>>>> I.e. the bits that need setting are in different registers, so you
>>>> have to have an if statement and separate calls. Therefore there's no
>>>> real benefit from the indirection you've introduced here, unless
>>>> you're expecting the SUN8I variant to use different bits in one of
>>>> those two registers.
>>>>
>>>
>>> That looks nice for this particular case.
>>
>> There was another case I pointed out in this driver that could
>> potentially benefit from this.
>>
>>> Still you will have to remember that these bits are specified directly
>>> while other bits on the register are mapped which is not so nice.
>>
>> True. I'm not sure which path is best in this case. To my eye, your
>> indirection scheme seems like the "heavy" solution, however I have no
>> idea what form a "lighter" solution would take.
>>
>> Other drivers I've seen which have tackled similar problems have used
>> a large struct to hold all the variant-specific stuff, whether that's
>> function pointers, register offsets, constants, etc. (so this code
>> could theoretically be re-written as sunxi_spi_write(sspi,
>> sspi->fifo_reg, reg | sspi->fifo_reset_arg)
>
> This won't do. There is fifo_reg and tfr_reg on both IPs but some bits
> from one migrated to the other.
>
>> or sspi->reset_fifo())
>
> This would work but would require more thorough rewrite and heavier
> adaptation layer.
>
> As it is where different register is used in different IP revision it
> is specified explicitly in the code while register names to numbers
> are mapped in read() and write(). Value bits are mapped by explicitly
> calling a function on the name.
>
> This gives nice 1:1 mapping to the old code and allows checking that
> both the register and the bit value in question exist on the IP.

And your method is almost beautiful from that perspective, however I
still feel that it's too "heavy". That said, neither of my suggestions
are much better. I provided them in the hope that they might be
illuminating.

> Matching the value to register relies on the driver code, however.

Of course.

I'm not sure what the state-of-the-art for dealing with variants of
devices where the manufacturer keeps scrambling the registers is,
however I feel we can do better, though I'm not sure how.

Thanks,

-- 
Julian Calaby

Email: julian.calaby@gmail.com
Profile: http://www.google.com/profiles/julian.calaby/