[PATCH v3] mmc: implement Driver Stage Register handling

[Uwe Kleine-König]

Hello Ulf,

On Thu, Aug 14, 2014 at 11:26:28AM +0200, Ulf Hansson wrote:
> On 13 August 2014 17:44, Uwe Kleine-König
> <u.kleine-koenig at pengutronix.de> wrote:
> > From: Sascha Hauer <s.hauer at pengutronix.de>
> >
> > Some (e)MMC and SD cards implement a DSR register that allows to tune
> > raise/fall times and drive strength of the CMD and DATA outputs.
> > The values to use depend on the card in use and the host.
> > It might be needed to reduce the drive strength to prevent voltage peaks
> > above the host's specification.
> >
> > Implement a 'dsr' devicetree property that allows to specify the value
> > to set the DSR to. For non-dt setups the new members of mmc_host can be
> > set by board code.
> >
> > This patch was initially authored by Sascha Hauer. It contains
> > improvements authored by Markus Niebel and Uwe Kleine-König.
> >
> > Signed-off-by: Sascha Hauer <s.hauer at pengutronix.de>
> > Signed-off-by: Markus Niebel <Markus.Niebel at tq-group.com>
> > Signed-off-by: Uwe Kleine-König <u.kleine-koenig at pengutronix.de>
> > ---
> > Hello,
> >
> > earlier incarnations of this patch can be found at
> >
> >         http://lists.infradead.org/pipermail/linux-arm-kernel/2014-July/272983.html
> >         http://lists.infradead.org/pipermail/linux-arm-kernel/2014-May/259281.html
> >
> > I need this functionallity on a machine where the default driver strength of
> > the eMMC chip is too big for the SoC. It seems to work without adapting the
> > drive strength, but the vendor reports that the DSR should be set to a certain
> > value to prevent poor signal integrity and increased wearout.
> >
> > Best regards
> > Uwe
> >
> >  Documentation/devicetree/bindings/mmc/mmc.txt |  2 ++
> >  drivers/mmc/core/host.c                       |  8 ++++++++
> >  drivers/mmc/core/mmc.c                        |  8 ++++++++
> >  drivers/mmc/core/mmc_ops.c                    | 21 +++++++++++++++++++++
> >  drivers/mmc/core/mmc_ops.h                    |  1 +
> >  drivers/mmc/core/sd.c                         |  8 ++++++++
> >  include/linux/mmc/card.h                      |  3 ++-
> >  include/linux/mmc/host.h                      |  3 +++
> >  8 files changed, 53 insertions(+), 1 deletion(-)
> >
> > diff --git a/Documentation/devicetree/bindings/mmc/mmc.txt b/Documentation/devicetree/bindings/mmc/mmc.txt
> > index 3c18001dfd5d..05bac770b4d0 100644
> > --- a/Documentation/devicetree/bindings/mmc/mmc.txt
> > +++ b/Documentation/devicetree/bindings/mmc/mmc.txt
> > @@ -40,6 +40,8 @@ Optional properties:
> >  - mmc-hs200-1_2v: eMMC HS200 mode(1.2V I/O) is supported
> >  - mmc-hs400-1_8v: eMMC HS400 mode(1.8V I/O) is supported
> >  - mmc-hs400-1_2v: eMMC HS400 mode(1.2V I/O) is supported
> > +- dsr: Value the card's (optional) Driver Stage Register (DSR) should be
> > +  programmed with.
> 
> Let's clarify that this is a 16 bit value.
ok.

> >  *NOTE* on CD and WP polarity. To use common for all SD/MMC host controllers line
> >  polarity properties, we have to fix the meaning of the "normal" and "inverted"
> > diff --git a/drivers/mmc/core/host.c b/drivers/mmc/core/host.c
> > index 95cceae96944..52e83f389428 100644
> > --- a/drivers/mmc/core/host.c
> > +++ b/drivers/mmc/core/host.c
> > @@ -452,6 +452,14 @@ int mmc_of_parse(struct mmc_host *host)
> >         if (of_find_property(np, "mmc-hs400-1_2v", &len))
> >                 host->caps2 |= MMC_CAP2_HS400_1_2V | MMC_CAP2_HS200_1_2V_SDR;
> >
> > +       if (of_find_property(np, "dsr", &len)) {
> > +               u32 tmp;
> > +
> > +               of_property_read_u32(np, "dsr", &tmp);
> > +               host->dsr_req = 1;
> > +               host->dsr = (u16)tmp;
> > +       }
> > +
> 
> Let's simplify the above with just:
> of_property_read_u16(np, "dsr", &host->dsr);
ok.

> >         return 0;
> >
> >  out:
> > diff --git a/drivers/mmc/core/mmc.c b/drivers/mmc/core/mmc.c
> > index 793c6f7ddb04..fdc1ac1360c4 100644
> > --- a/drivers/mmc/core/mmc.c
> > +++ b/drivers/mmc/core/mmc.c
> > @@ -162,6 +162,7 @@ static int mmc_decode_csd(struct mmc_card *card)
> >         csd->read_partial = UNSTUFF_BITS(resp, 79, 1);
> >         csd->write_misalign = UNSTUFF_BITS(resp, 78, 1);
> >         csd->read_misalign = UNSTUFF_BITS(resp, 77, 1);
> > +       csd->dsr_imp = UNSTUFF_BITS(resp, 76, 1);
> >         csd->r2w_factor = UNSTUFF_BITS(resp, 26, 3);
> >         csd->write_blkbits = UNSTUFF_BITS(resp, 22, 4);
> >         csd->write_partial = UNSTUFF_BITS(resp, 21, 1);
> > @@ -1273,6 +1274,13 @@ static int mmc_init_card(struct mmc_host *host, u32 ocr,
> >         }
> >
> >         /*
> > +        * handling only for cards supporting DSR and hosts requesting
> > +        * DSR configuration
> > +        */
> > +       if (card->csd.dsr_imp && host->dsr_req)
> 
> We don't need host->dsr_req. Instead just check host->dsr.
I think this doesn't work. What is your actual suggestion?

	if (card->csd.dsr_imp && host->dsr)

? The intended semantic is that if the device tree has:

	dsr = <$somevalue>;

the DSR is written, and if there is no such property, DSR is unhandled.
If you just check for host->dsr being != 0, how to differenciate between

	dsr = <0>;

in the device tree and dsr not being specified?

Best regards
Uwe

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