linux-kernel - Re: [PATCH v3 06/13] mmc: mmci: Qcomm: Add 3 clock cycle delay after register write

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Date:	Mon, 26 May 2014 11:34:37 +0200
From:	Ulf Hansson <ulf.hansson@...aro.org>
To:	Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
Cc:	Russell King <linux@....linux.org.uk>,
	linux-mmc <linux-mmc@...r.kernel.org>,
	Chris Ball <chris@...ntf.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-arm-msm@...r.kernel.org,
	Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [PATCH v3 06/13] mmc: mmci: Qcomm: Add 3 clock cycle delay after
 register write

On 23 May 2014 14:51,  <srinivas.kandagatla@...aro.org> wrote:
> From: Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
>
> Most of the Qcomm SD card controller registers must be updated to the MCLK
> domain so subsequent writes to registers will be ignored until 3 clock cycles
> have passed.
>
> This patch adds a 3 clock cycle delay required after writing to controller
> registers on Qualcomm SOCs. Without this delay all the register writes are not
> successful, resulting in not detecting cards. The write clock delay is
> activated by setting up mclk_delayed_writes variable in variant data.
>
> Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
> Reviewed-by: Linus Walleij <linus.walleij@...aro.org>
> ---
>  drivers/mmc/host/mmci.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/drivers/mmc/host/mmci.c b/drivers/mmc/host/mmci.c
> index 881bb24..1385554 100644
> --- a/drivers/mmc/host/mmci.c
> +++ b/drivers/mmc/host/mmci.c
> @@ -67,6 +67,8 @@ static unsigned int fmax = 515633;
>   * @pwrreg_clkgate: MMCIPOWER register must be used to gate the clock
>   * @busy_detect: true if busy detection on dat0 is supported
>   * @pwrreg_nopower: bits in MMCIPOWER don't controls ext. power supply
> + * @mclk_delayed_writes: enable delayed writes to ensure, subsequent updates
> + *                      are not ignored.
>   */
>  struct variant_data {
>         unsigned int            clkreg;
> @@ -83,6 +85,7 @@ struct variant_data {
>         bool                    pwrreg_clkgate;
>         bool                    busy_detect;
>         bool                    pwrreg_nopower;
> +       bool                    mclk_delayed_writes;
>  };
>
>  static struct variant_data variant_arm = {
> @@ -171,6 +174,12 @@ static struct variant_data variant_qcom = {
>         .datalength_bits        = 24,
>         .blksz_datactrl4        = true,
>         .pwrreg_powerup         = MCI_PWR_UP,
> +       /*
> +        * On QCom SD card controller, registers must be updated to the
> +        * MCLK domain so subsequent writes to this register will be ignored
> +        * for 3 clk cycles.
> +        */
> +       .mclk_delayed_writes    = true,
>  };
>
>  static inline u32 mmci_readl(struct mmci_host *host, u32 off)
> @@ -181,6 +190,9 @@ static inline u32 mmci_readl(struct mmci_host *host, u32 off)
>  static inline void mmci_writel(struct mmci_host *host, u32 data, u32 off)
>  {
>         writel(data, host->base + off);
> +
> +       if (host->variant->mclk_delayed_writes)
> +               udelay(DIV_ROUND_UP((3 * USEC_PER_SEC), host->mclk));
>  }

I am not sure I like this approach. For each and every writel
(including pio_writes) you will add a few cpu cycles, since you need
to  check for "mclk_delayed_writes" no matter of variant.

How about, adding a new function pointer in the struct mmci_host, for
"writel operations" which you could set up in probe phase instead?

Kind regards
Ulf Hansson
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