[PATCHv2 01/31] drm/omap: work-around for errata i886

[Tomi Valkeinen]

On 28/03/17 08:45, Jyri Sarha wrote:
> On 03/24/17 11:40, Tomi Valkeinen wrote:
>> DRA7 errata i886 (FPDLink PLL Unlocks With Certain SoC PLL M/N Values)
>> says that FPDLink is sensitive to jitter on the vout clock, and that low
>> PLL M and N values result in more jitter than high M and N values.
>>
>> This patch implements a workaround for the problem by changing the PLL
>> setup to search for clocks starting from high M and N values, instead of
>> low values. This should not cause any functional change, and only
>> reduces the jitter.
>>
>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen at ti.com>
>> ---
>>  drivers/gpu/drm/omapdrm/dss/pll.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/omapdrm/dss/pll.c b/drivers/gpu/drm/omapdrm/dss/pll.c
>> index 0a76c89cdc2e..65c478d85caa 100644
>> --- a/drivers/gpu/drm/omapdrm/dss/pll.c
>> +++ b/drivers/gpu/drm/omapdrm/dss/pll.c
>> @@ -231,7 +231,7 @@ bool dss_pll_calc_a(const struct dss_pll *pll, unsigned long clkin,
>>  
>>  	pll_max = pll_max ? pll_max : ULONG_MAX;
>>  
>> -	for (n = n_start; n <= n_stop; ++n) {
>> +	for (n = n_stop; n >= n_start; --n) {
> 
> The patch looks simple like this, but just reading the code without a
> hint what is going on may be confusing. Loops beginning from stop and
> ending in start and all.
> 
> Could these variables be called [nm]_max and [nm]_min, or maybe one
> simple comment to tell why we are going from max to min, or both?

Indeed, it's confusing. I did both.

 Tomi

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