Re: [PATCH 2/3] fpga manager: xilinx-spi: provide better diagnostics on programming failure

[Tom Rix]

On 8/18/20 3:20 AM, Luca Ceresoli wrote:
> [a question for GPIO maintainers below]
>
> Hi Tom,
>
> thanks for your review!
>
> On 17/08/20 20:15, Tom Rix wrote:
>> The other two patches are fine.
>>
>> On 8/17/20 9:59 AM, Luca Ceresoli wrote:
>>> When the DONE pin does not go high after programming to confirm programming
>>> success, the INIT_B pin provides some info on the reason. Use it if
>>> available to provide a more explanatory error message.
>>>
>>> Signed-off-by: Luca Ceresoli <luca@xxxxxxxxxxxxxxxx>
>>> ---
>>>  drivers/fpga/xilinx-spi.c | 11 ++++++++++-
>>>  1 file changed, 10 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/fpga/xilinx-spi.c b/drivers/fpga/xilinx-spi.c
>>> index 502fae0d1d85..2aa942bb1114 100644
>>> --- a/drivers/fpga/xilinx-spi.c
>>> +++ b/drivers/fpga/xilinx-spi.c
>>> @@ -169,7 +169,16 @@ static int xilinx_spi_write_complete(struct fpga_manager *mgr,
>>>  			return xilinx_spi_apply_cclk_cycles(conf);
>>>  	}
>>>  
>>> -	dev_err(&mgr->dev, "Timeout after config data transfer.\n");
>>> +	if (conf->init_b) {
>>> +		int init_b_asserted = gpiod_get_value(conf->init_b);
>> gpiod_get_value can fail. So maybe need split the first statement.
>>
>> init_b_asserted < 0 ? "invalid device"
>>
>> As the if-else statement is getting complicated, embedding the ? : makes this hard to read.  'if,else if, else' would be better.
> Thanks for the heads up. However I'm not sure which is the best thing to
> do here.
>
> First, I've been reading the libgpiod code after your email and yes, the
> libgpiod code _could_ return runtime errors received from the gpiochip
> driver, even though the docs state:
>
>> The get/set calls do not return errors because “invalid GPIO”> should have been reported earlier from gpiod_direction_*().
> (https://www.kernel.org/doc/html/latest/driver-api/gpio/consumer.html)
>
> On the other hand there are plenty of calls to gpiod_get/set_value in
> the kernel that don't check for error values. I guess this is because
> failures getting/setting a GPIO are very uncommon (perhaps impossible
> with platform GPIO).
>
> When still a GPIO get/set operation fails I'm not sure adding thousands
> of error-checking code lines in hundreds of drivers is the best way to
> go. I feel like we should have a unique, noisy dev_err() in the error
> path in libgpio but I was surprised in not finding any [1].
>
> Linus, Bartosz, what's your opinion? Should all drivers check for errors
> after every gpiod_[sg]et_value*() call?

My opinion is that you know the driver / hw is in a bad state and you

are trying to convey useful information.  So you should

be as careful as possible and not assume gpio did not fail.

>
>>> +		dev_err(&mgr->dev,
>>> +			init_b_asserted ? "CRC error or invalid device\n"
>>> +			: "Missing sync word or incomplete bitstream\n");
>>> +	} else {
>>> +		dev_err(&mgr->dev, "Timeout after config data transfer.\n");
>> patch 3 removes '.' s , and you just added one back in ?
> Here I'm only changing indentation of this line. But OK, this is
> misleading, so I'll swap patches 2 and 3 in the next patch iteration to
> avoid confusion.
Maybe just remove the '.' at the same time and/or collapse 2&3 into a single patch.
>
> [1]
> https://elixir.bootlin.com/linux/v5.8/source/drivers/gpio/gpiolib.c#L3646
>