linux-kernel - Re: [PATCH v2 3/5] fpga manager: xilinx-spi: rework write

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Date:   Fri, 28 Aug 2020 05:34:02 -0700
From:   Tom Rix <trix@...hat.com>
To:     Luca Ceresoli <luca@...aceresoli.net>
Cc:     linux-fpga@...r.kernel.org, Moritz Fischer <mdf@...nel.org>,
        Michal Simek <michal.simek@...inx.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Anatolij Gustschin <agust@...x.de>
Subject: Re: [PATCH v2 3/5] fpga manager: xilinx-spi: rework write_complete
 loop implementation


On 8/27/20 11:38 PM, Luca Ceresoli wrote:
> Hi Tom,
>
> On 27/08/20 21:34, Tom Rix wrote:
>> On 8/27/20 12:26 PM, Luca Ceresoli wrote:
>>> Hi Tom,
>>>
>>> thanks for the prompt feedback!
>>>
>>> On 27/08/20 20:59, Tom Rix wrote:
>>>> On 8/27/20 7:32 AM, Luca Ceresoli wrote:
>>>>> In preparation to add error checking for gpiod_get_value(), rework
>>>>> the loop to avoid the duplication of these lines:
>>>>>
>>>>> 	if (gpiod_get_value(conf->done))
>>>>> 		return xilinx_spi_apply_cclk_cycles(conf);
>>>>>
>>>>> There is little advantage in this rework with current code. However
>>>>> error checking will expand these two lines to five, making code
>>>>> duplication more annoying.
>>>>>
>>>>> Signed-off-by: Luca Ceresoli <luca@...aceresoli.net>
>>>>>
>>>>> ---
>>>>>
>>>>> This patch is new in v2
>>>>> ---
>>>>>  drivers/fpga/xilinx-spi.c | 15 ++++++---------
>>>>>  1 file changed, 6 insertions(+), 9 deletions(-)
>>>>>
>>>>> diff --git a/drivers/fpga/xilinx-spi.c b/drivers/fpga/xilinx-spi.c
>>>>> index 01f494172379..cfc933d70f52 100644
>>>>> --- a/drivers/fpga/xilinx-spi.c
>>>>> +++ b/drivers/fpga/xilinx-spi.c
>>>>> @@ -151,22 +151,19 @@ static int xilinx_spi_write_complete(struct fpga_manager *mgr,
>>>>>  				     struct fpga_image_info *info)
>>>>>  {
>>>>>  	struct xilinx_spi_conf *conf = mgr->priv;
>>>>> -	unsigned long timeout;
>>>>> +	unsigned long timeout = jiffies + usecs_to_jiffies(info->config_complete_timeout_us);
>>>>>  	int ret;
>>>>>  
>>>>> -	if (gpiod_get_value(conf->done))
>>>>> -		return xilinx_spi_apply_cclk_cycles(conf);
>>>>> -
>>>>> -	timeout = jiffies + usecs_to_jiffies(info->config_complete_timeout_us);
>>>>> +	while (true) {
>>>>> +		if (gpiod_get_value(conf->done))
>>>>> +			return xilinx_spi_apply_cclk_cycles(conf);
>>>>>  
>>>>> -	while (time_before(jiffies, timeout)) {
>>>>> +		if (time_after(jiffies, timeout))
>>>>> +			break;
>>>>>  
>>>>>  		ret = xilinx_spi_apply_cclk_cycles(conf);
>>>>>  		if (ret)
>>>>>  			return ret;
>>>>> -
>>>>> -		if (gpiod_get_value(conf->done))
>>>>> -			return xilinx_spi_apply_cclk_cycles(conf);
>>>>>  	} 
>>>> Do you need another
>>>>
>>>> 	if (gpiod_get_value(conf->done))
>>>> 		return xilinx_spi_apply_cclk_cycles(conf);
>>>>
>>>> here to cover the chance of sleeping in the loop ?
>>> If I got your question correctly: if we get here it's because of a
>>> timeout, thus programming has failed (DONE didn't come up after some
>>> time), and checking it one more here seems pointless.
>> It may not be pointless, if this routine sleeps because it was scheduled out, when it wakes up a lot of time  happened. You will see this as a timeout but the state may be good.  Another, final check at the end will cover this case.
> Oh, now I got your point! Yes, there is this risk, and it exists in
> current code as well but with a smaller risk window. Unrolling the
> current and new loop code they behave the same except for the position
> of the timeout computation (after vs before the first 'if (done) return'
> group).
>
> I think this reimplementation is sleep-safe, check for GPIO errors and
> also avoid code duplication:
>
> static int xilinx_spi_write_complete(struct fpga_manager *mgr,
> 				     struct fpga_image_info *info)
> {
> 	struct xilinx_spi_conf *conf = mgr->priv;
> 	unsigned long timeout = jiffies +
> 		usecs_to_jiffies(info->config_complete_timeout_us);
> 	bool expired;
> 	int done;
> 	int ret;
>
> 	while (!expired) {
> 		expired = time_after(jiffies, timeout);
>
> 		done = get_done_gpio(mgr);
> 		if (done < 0)
> 			return done;
>
> 		ret = xilinx_spi_apply_cclk_cycles(conf);
> 		if (ret)
> 			return ret;
>
> 		if (done)
> 			return 0;
> 	}
>
> 	dev_err(&mgr->dev, "Timeout after config data transfer\n");
>
> 	return -ETIMEDOUT;
> }
>
> A key point is to assess all the status (expired and done variables)
> before taking any action based on it. Then we can unconditionally apply
> 8 cclk cycles before even checking the actual DONE value, so that we
> always do that after DONE has been seen asserted.
>
> Does it look good?

Yes. Thanks for the extra work.

Tom

>