[U-Boot] [PATCH v1] mmc: dwmmc: Enable small delay before returning error

[Ang, Chee Hong <chee.hong.ang@intel.com>]

On Mon, 2019-02-18 at 21:38 +0100, Marek Vasut wrote:
> On 2/18/19 3:51 PM, Ang, Chee Hong wrote:
> > 
> > On Mon, 2019-02-18 at 12:57 +0100, Marek Vasut wrote:
> > > 
> > > On 2/18/19 5:16 AM, chee.hong.ang at intel.com wrote:
> > > > 
> > > > 
> > > > From: "Ang, Chee Hong" <chee.hong.ang@intel.com>
> > > > 
> > > > 'SET_BLOCKLEN' may occasionally fail on first attempt.
> > > Why ?
> > This is part of the workaround of mmc driver which is enabled with
> > 'CONFIG_MMC_QUIRKS' config.
> > https://github.com/u-boot/u-boot/blob/dbe70c7d4e3d5c705a98d82952e05
> > a591
> > efd0683/drivers/mmc/mmc.c#L272
> OK, let's take a step back. What problem do you observe, that you're
> trying to fix ?
See my detail explanation below.
> 
> > 
> > > 
> > > > 
> > > > This patch enable a small delay in dwmci_send_cmd() on
> > > > busy, I/O or CRC error to allow the MMC controller recovers
> > > > from the failure/error on subsequent retries.
> > > Why 100 uS ?
> > This is a good question. Perhaps the driver's authors can explain
> > this.
> > The driver delay 100us after dwmci_send_cmd() success with the
> > command
> > sent. But it never delay 100us whenever mmc controller response to
> > the
> > command sent with I/O or CRC errors. MMC drivers expect the first
> > 'SET_BLOCKEN' command issued by dwmci_send_cmd() may fail
> > intermittently so it will retry up to 4 times before it gave up and
> > return error. Without this 100us delay after error response,
> > 'SET_BLOCKEN' may sometime fail in 4 attempts in a row. Therefore,
> > we
> > encountered intermittent failure in loading u-boot (SSBL) from MMC.
> Can you be more specific about the failure you saw ?

Here are the steps for booting u-boot (SSBL) from MMC:

1) SPL (FSBL) get loaded to OCRAM
2) SPL try to load uboot as SSBL from MMC
3) SPL issue 'SET_BLOCKEN' to MMC controller
4) If MMC controller response with error continue to step 5 else go to
step 7
5) If is less than 4 attempts go to step 3 else continue to step 6
6) Indicate to user there was an error loading uboot as SSBL from MMC
and stop here
7) Send command to read blocks of data from MMC
8) uboot successfully loaded to DDR memory
9) SPL jumps to uboot and continue the boot flow.

So this patch actually introduce a small delay at step 4. Without this
small delay, step 4 might sometime fail in all 4 attempts to issue the
'SET_BLOCKEN' command to MMC controller. Note that this failure is
intermittent. If it fails at step 3 and there is no small delay in step
4, it will most likely fail in subsequent attempts.
> 
> > 
> > > 
> > > Can we rather detect whether the controller recovered by polling
> > > some
> > > bit?
> > Hmmm...I can take a look at the databook of this controller and dig
> > further. Probably this is the limitation of the controller itself.
> > The
> > mmc driver code which introduce 100us delay after every command
> > sent in
> > dwmci_send_cmd() looks iffy.
> If you have access to it, please do.
Unfortunately, I can't find any registers I can poll for the status of
the readiness of the controller from DesignWare storage controller
databook.
> 
> btw do you have the same problem if you disable caches ?
The error happen while SPL trying to load uboot as SSBL from MMC into
DDR memory.At this point, the caches are disabled.
> 
> > 
> > > 
> > > > 
> > > > Signed-off-by: Ang, Chee Hong <chee.hong.ang@intel.com>
> > > > ---
> > > >  drivers/mmc/dw_mmc.c | 14 ++++++++++----
> > > >  1 file changed, 10 insertions(+), 4 deletions(-)
> > > > 
> > > > diff --git a/drivers/mmc/dw_mmc.c b/drivers/mmc/dw_mmc.c
> > > > index 7544b84..8dcc518 100644
> > > > --- a/drivers/mmc/dw_mmc.c
> > > > +++ b/drivers/mmc/dw_mmc.c
> > > > @@ -266,8 +266,11 @@ static int dwmci_send_cmd(struct mmc *mmc,
> > > > struct mmc_cmd *cmd,
> > > >  	if (data)
> > > >  		flags = dwmci_set_transfer_mode(host, data);
> > > >  
> > > > -	if ((cmd->resp_type & MMC_RSP_136) && (cmd->resp_type
> > > > &
> > > > MMC_RSP_BUSY))
> > > > -		return -1;
> > > > +	if ((cmd->resp_type & MMC_RSP_136) &&
> > > > +	    (cmd->resp_type & MMC_RSP_BUSY)) {
> > > > +		ret = -1;
> > > > +		goto delay_ret;
> > > > +	}
> > > >  
> > > >  	if (cmd->cmdidx == MMC_CMD_STOP_TRANSMISSION)
> > > >  		flags |= DWMCI_CMD_ABORT_STOP;
> > > > @@ -316,11 +319,13 @@ static int dwmci_send_cmd(struct mmc
> > > > *mmc,
> > > > struct mmc_cmd *cmd,
> > > >  		return -ETIMEDOUT;
> > > >  	} else if (mask & DWMCI_INTMSK_RE) {
> > > >  		debug("%s: Response Error.\n", __func__);
> > > > -		return -EIO;
> > > > +		ret = -EIO;
> > > > +		goto delay_ret;
> > > >  	} else if ((cmd->resp_type & MMC_RSP_CRC) &&
> > > >  		   (mask & DWMCI_INTMSK_RCRC)) {
> > > >  		debug("%s: Response CRC Error.\n", __func__);
> > > > -		return -EIO;
> > > > +		ret = -EIO;
> > > > +		goto delay_ret;
> > > >  	}
> > > >  
> > > >  
> > > > @@ -347,6 +352,7 @@ static int dwmci_send_cmd(struct mmc *mmc,
> > > > struct mmc_cmd *cmd,
> > > >  		}
> > > >  	}
> > > >  
> > > > +delay_ret:
> > > >  	udelay(100);
> > > >  
> > > >  	return ret;
> > > > 
>