linux-kernel - Re: [PATCH 2/2] mmc: core: Add support for cache ctrl for SD cards

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Date:   Sun, 9 May 2021 21:01:27 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     linux-mmc <linux-mmc@...r.kernel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Shawn Lin <shawn.lin@...k-chips.com>,
        Avri Altman <avri.altman@....com>,
        Masami Hiramatsu <masami.hiramatsu@...aro.org>,
        linux-block <linux-block@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mmc: core: Add support for cache ctrl for SD cards

On Thu, May 6, 2021 at 4:58 PM Ulf Hansson <ulf.hansson@...aro.org> wrote:

> In SD spec v6.x the SD function extension registers for performance
> enhancements were introduced. As a part of this an optional internal cache
> on the SD card, can be used to improve performance.
>
> The let the SD card use the cache, the host needs to enable it and manage
> flushing of the cache, so let's add support for this.
>
> Note that for an SD card supporting the cache it's mandatory for it, to
> also support the poweroff notification feature. According to the SD spec,
> if the cache has been enabled and a poweroff notification is sent to the
> card, that implicitly also means that the card should flush its internal
> cache. Therefore, dealing with cache flushing for REQ_OP_FLUSH block
> requests is sufficient.
>
> Signed-off-by: Ulf Hansson <ulf.hansson@...aro.org>
(...)

> +       /*
> +        * Set the Flush Cache bit in the performance enhancement register at
> +        * 261 bytes offset.
> +        */
> +       fno = card->ext_perf.fno;
> +       page = card->ext_perf.page;
> +       offset = card->ext_perf.offset + 261;

261 looks a bit magic, can we add a define of some sort?
I guess it has a name in the spec?

> +       err = sd_write_ext_reg(card, fno, page, offset, 0x1);
> +       if (err) {
> +               pr_warn("%s: error %d writing Cache Flush bit\n",
> +                       mmc_hostname(host), err);
> +               goto out;
> +       }

So this offset contains a single bit.

> +       if (reg_buf[0] & 0x1)
> +               err = -ETIMEDOUT;

And that same bit is checked here.

Is it always going to be one bit only or do we want to

#include <linux/bits.h>
#define SD_CACHE_FLUSH_FLAG BIT(0)

Does it have a name in the spec we can use?

> +       /*
> +        * Set the Cache Enable bit in the performance enhancement register at
> +        * 260 bytes offset.
> +        */
> +       err = sd_write_ext_reg(card, card->ext_perf.fno, card->ext_perf.page,
> +                              card->ext_perf.offset + 260, 0x1);

Same here we want to #define 260 to something symbolic,

And here some define for BIT(0) as well. At least with BIT(0)
in the call to sd_write_ext_reg() rather than 0x1 if I can say
something.

With the above nitpicking fixed up (I trust you):
Reviewed-by: Linus Walleij <linus.walleij@...aro.org>

Yours,
Linus Walleij