Re: [PATCH v2 7/9] spi: spi-s3c64xx: Ensure cur_speed holds actual clock value

From: Lukasz Stelmach <l.stelmach@samsung.com>
To: Tomasz Figa <tfiga@chromium.org>
Cc: Krzysztof Kozlowski <krzk@kernel.org>,
	Kukjin Kim <kgene@kernel.org>, Andi Shyti <andi@etezian.org>,
	Mark Brown <broonie@kernel.org>,
	linux-spi@vger.kernel.org,
	linux-samsung-soc <linux-samsung-soc@vger.kernel.org>,
	"list\@263.net\:IOMMU DRIVERS
	\<iommu\@lists.linux-foundation.org\>\,
	Joerg Roedel \<joro\@8bytes.org\>\," 
	<linux-arm-kernel@lists.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Marek Szyprowski <m.szyprowski@samsung.com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Subject: Re: [PATCH v2 7/9] spi: spi-s3c64xx: Ensure cur_speed holds actual clock value
Date: Tue, 25 Aug 2020 11:01:53 +0200	[thread overview]
Message-ID: <dleftjk0xnw132.fsf%l.stelmach@samsung.com> (raw)
In-Reply-To: <CAAFQd5ADym6YapCoJ8+fJbPjSestcD_2R8L5T8jAfO4c=GFQkA@mail.gmail.com> (Tomasz Figa's message of "Mon, 24 Aug 2020 15:21:34 +0200")

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It was <2020-08-24 pon 15:21>, when Tomasz Figa wrote:
> On Mon, Aug 24, 2020 at 3:17 PM Lukasz Stelmach <l.stelmach@samsung.com> wrote:
>>
>> It was <2020-08-22 sob 14:43>, when Krzysztof Kozlowski wrote:
>> > On Fri, Aug 21, 2020 at 06:13:59PM +0200, Łukasz Stelmach wrote:
>> >> cur_speed is used to calculate transfer timeout and needs to be
>> >> set to the actual value of (half) the clock speed for precise
>> >> calculations.
>> >
>> > If you need this only for timeout calculation just divide it in
>> > s3c64xx_wait_for_dma().
>>
>> I divide it here to keep the relationship between the value the variable
>> holds and the one that is inside clk_* (See? It's multiplied 3 lines
>> above). If you look around every single clk_get_rate() call in the file is
>> divided by two.
>>
>> > Otherwise why only if (cmu) case is updated?
>>
>> You are righ I will update that too.
>>
>> However, I wonder if it is even possible that the value read from
>> S3C64XX_SPI_CLK_CFG would be different than the one written to it?
>>
>
> It is not possible for the register itself, but please see my other
> reply, where I explained the integer rounding error which can happen
> when calculating the value to write to the register.

I don't have any board to test it and Marek says there is only one that
doesn't use cmu *and* has an SPI device attached.

Here is what I think should work for the !cmu case.

--8<---------------cut here---------------start------------->8---

diff --git a/drivers/spi/spi-s3c64xx.c b/drivers/spi/spi-s3c64xx.c
index 18b89e53ceda..5ebb1caade4d 100644
--- a/drivers/spi/spi-s3c64xx.c
+++ b/drivers/spi/spi-s3c64xx.c
@@ -655,13 +655,18 @@ static int s3c64xx_spi_config(struct
s3c64xx_spi_driver_data *sdd)
                        return ret;
                sdd->cur_speed = clk_get_rate(sdd->src_clk) / 2;
        } else {
+               int src_clk_rate = clk_get_rate(sdd->src_clk);
+               int clk_val = (src_clk_rate / sdd->cur_speed / 2 - 1);
+
                /* Configure Clock */
                val = readl(regs + S3C64XX_SPI_CLK_CFG);
                val &= ~S3C64XX_SPI_PSR_MASK;
-               val |= ((clk_get_rate(sdd->src_clk) / sdd->cur_speed / 2 - 1)
-                               & S3C64XX_SPI_PSR_MASK);
+               val |= (clk_val & S3C64XX_SPI_PSR_MASK);
                writel(val, regs + S3C64XX_SPI_CLK_CFG);

+               /* Keep the actual value */
+               sdd->cur_speed = src_clk_rate / (2 * (clk_val + 1));
+
                /* Enable Clock */
                val = readl(regs + S3C64XX_SPI_CLK_CFG);
                val |= S3C64XX_SPI_ENCLK_ENABLE;
--8<---------------cut here---------------end--------------->8---


>> > You are also affecting here not only timeout but
>> > s3c64xx_enable_datapath() which is not mentioned in commit log. In other
>> > words, this looks wrong.
>>
>> Indeed, there is a reference too. I've corrected the message.
>>
>
> Thanks!
>
> Best regards,
> Tomasz
>
>> >>
>> >> Cc: Tomasz Figa <tfiga@chromium.org>
>> >> Signed-off-by: Łukasz Stelmach <l.stelmach@samsung.com>
>> >> ---
>> >>  drivers/spi/spi-s3c64xx.c | 1 +
>> >>  1 file changed, 1 insertion(+)
>> >>
>> >> diff --git a/drivers/spi/spi-s3c64xx.c b/drivers/spi/spi-s3c64xx.c
>> >> index 02de734b8ab1..89c162efe355 100644
>> >> --- a/drivers/spi/spi-s3c64xx.c
>> >> +++ b/drivers/spi/spi-s3c64xx.c
>> >> @@ -626,6 +626,7 @@ static int s3c64xx_spi_config(struct s3c64xx_spi_driver_data *sdd)
>> >>              ret = clk_set_rate(sdd->src_clk, sdd->cur_speed * 2);
>> >>              if (ret)
>> >>                      return ret;
>> >> +            sdd->cur_speed = clk_get_rate(sdd->src_clk) / 2;
>> >>      } else {
>> >>              /* Configure Clock */
>> >>              val = readl(regs + S3C64XX_SPI_CLK_CFG);
>> >> --
>> >> 2.26.2
>> >>
>> >
>> >
>>
>> --
>> Łukasz Stelmach
>> Samsung R&D Institute Poland
>> Samsung Electronics
>
>

-- 
Łukasz Stelmach
Samsung R&D Institute Poland
Samsung Electronics

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