linux-kernel - Re: [RFC][PATCH 2/2] spi: s3c64xx: Use clkdev for bus clock lookup

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Date:	Wed, 10 Aug 2011 17:33:36 +0530
From:	padma venkat <padma.kvr@...il.com>
To:	Jassi Brar <jassisinghbrar@...il.com>
Cc:	Padmavathi Venna <padma.v@...sung.com>, kgene.kim@...sung.com,
	linux@....linux.org.uk, linux-arm-kernel@...ts.infradead.org,
	linux-samsung-soc@...r.kernel.org, linux-kernel@...r.kernel.org,
	grant.likely@...retlab.ca
Subject: Re: [RFC][PATCH 2/2] spi: s3c64xx: Use clkdev for bus clock lookup

Hi Jassi,

On Tue, Aug 9, 2011 at 6:13 PM, Jassi Brar <jassisinghbrar@...il.com> wrote:
> On Tue, Aug 9, 2011 at 7:28 PM, Padmavathi Venna <padma.v@...sung.com> wrote:
>> SPI driver is modified to lookup the bus clock using the
>> alias name instead of getting clock name and clock
>> number from platform data.
> Cool.
>
>> Driver is modified to get the best source clock among the
>> available source clocks for the required frequency.
> I am not sure if this driver should be deciding which clock is 'best' for it.
> Because ...
> 1) Usually it's the board designer who decides which clock to run at
>  what speed based upon target device. So ideally, based upon use-case
> the driver should simply get the 'best' clock from board via platform in a
> format that is compliant to the 'generic clock api'.
As per your comment I modified the code for board designer to supply the
required list of source clocks. If this list is NULL then it  uses the
all available
clock sources. I will resubmit this patch.
> 2) We are not changing source clock rates(and we should not).
>  So the 'best' clock found, might still be way off in accuracy. And when we
> can't anyway guarantee accuracy, why not leave the decision to
> the board designer who might, say, select the source clock good enough
> to be useful to more than one controller yet not absolutely accurate for any ?
Yes. We are not getting the accurate frequency but we are able to get
best frequency.
> 3) It keeps enabled 3 unused clocks all the time.
I modified the  code to enable only the best source clock. I will
resubmit this patch.
>
>
>> diff --git a/drivers/spi/spi_s3c64xx.c b/drivers/spi/spi_s3c64xx.c
>> index 8945e20..d7c979d 100644
>> --- a/drivers/spi/spi_s3c64xx.c
>> +++ b/drivers/spi/spi_s3c64xx.c
>> @@ -132,6 +132,9 @@
>>  #define RXBUSY    (1<<2)
>>  #define TXBUSY    (1<<3)
>>
>> +#define MAX_SPI_BUS_CLK (4)
>> +#define MAX_PSR (256)
>
> Btw,
>
>  #define MAX_SPI_BUS_CLK 4
>  #define MAX_PSR 256
>
> is safe ... even safer than with two on!
Modified.
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