linux-kernel - Re: [PATCH] drm/ast: astdp: fix pre-op vs post-op bug

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Message-ID: <87h6bt3j6w.fsf@intel.com>
Date: Fri, 09 Aug 2024 16:43:51 +0300
From: Jani Nikula <jani.nikula@...ux.intel.com>
To: Thomas Zimmermann <tzimmermann@...e.de>, Dan Carpenter
 <dan.carpenter@...aro.org>
Cc: Dave Airlie <airlied@...hat.com>, Jocelyn Falempe <jfalempe@...hat.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard
 <mripard@...nel.org>, David Airlie <airlied@...il.com>, Daniel Vetter
 <daniel@...ll.ch>, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] drm/ast: astdp: fix pre-op vs post-op bug

On Fri, 09 Aug 2024, Thomas Zimmermann <tzimmermann@...e.de> wrote:
> Hi,
>
> thanks a lot for the bugfix.
>
> Am 09.08.24 um 14:33 schrieb Dan Carpenter:
>> The test for "Link training failed" expect the loop to exit with "i"
>> set to zero but it exits when "i" is set to -1.  Change this from a
>> post-op to a pre-op so that it exits with "i" set to zero.  This
>> changes the number of iterations from 10 to 9 but probably that's
>> okay.
>
> Yes, that's ok.
>
>>
>> Fixes: 2281475168d2 ("drm/ast: astdp: Perform link training during atomic_enable")
>> Signed-off-by: Dan Carpenter <dan.carpenter@...aro.org>
>> ---
>>   drivers/gpu/drm/ast/ast_dp.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/ast/ast_dp.c b/drivers/gpu/drm/ast/ast_dp.c
>> index 5d07678b502c..4329ab680f62 100644
>> --- a/drivers/gpu/drm/ast/ast_dp.c
>> +++ b/drivers/gpu/drm/ast/ast_dp.c
>> @@ -148,7 +148,7 @@ void ast_dp_link_training(struct ast_device *ast)
>>   	struct drm_device *dev = &ast->base;
>>   	unsigned int i = 10;
>>   
>> -	while (i--) {
>> +	while (--i) {
>
> If this loop ever starts with i = 0, it would break again. Can we use
>
> while (i) {
>    --i;
>     ...
> }
>
> instead?

FWIW, I personally *always* use for loops when there isn't a compelling
reason to do otherwise. You know at a glance that

	for (i = 0; i < N; i++)

gets run N times and what i is going to be afterwards.

Sure, you may have to restructure other things, but I think it's almost
always worth it.

BR,
Jani.



>
> Best regards
> Thomas
>
>>   		u8 vgacrdc = ast_get_index_reg(ast, AST_IO_VGACRI, 0xdc);
>>   
>>   		if (vgacrdc & AST_IO_VGACRDC_LINK_SUCCESS)

-- 
Jani Nikula, Intel