From: Boris Brezillon <boris.brezillon@collabora.com>
To: Pratyush Yadav <me@yadavpratyush.com>
Cc: vigneshr@ti.com, tudor.ambarus@microchip.com, richard@nod.at,
john.garry@huawei.com, linux-spi@vger.kernel.org,
broonie@kernel.org, linux-mtd@lists.infradead.org,
miquel.raynal@bootlin.com, Yicong Yang <yangyicong@hisilicon.com>
Subject: Re: [RFC PATCH 3/3] spi: hisi-sfc-v3xx: Add prepare/unprepare methods to avoid race condition
Date: Tue, 26 May 2020 11:30:16 +0200 [thread overview]
Message-ID: <20200526113016.58e6687a@collabora.com> (raw)
In-Reply-To: <20200526112752.6dd0da2c@collabora.com>
On Tue, 26 May 2020 11:27:52 +0200
Boris Brezillon <boris.brezillon@collabora.com> wrote:
> On Mon, 25 May 2020 21:44:36 +0530
> Pratyush Yadav <me@yadavpratyush.com> wrote:
>
> > Hi Yicong,
> >
> > On 21/05/20 07:23PM, Yicong Yang wrote:
> > > The controller can be shared with the firmware, which may cause race
> > > problems. As most read/write/erase/lock/unlock of spi-nor flash are
> > > composed of a set of operations, while the firmware may use the controller
> > > and start its own operation in the middle of the process started by the
> > > kernel driver, which may lead to the kernel driver's function broken.
> > >
> > > Bit[20] in HISI_SFC_V3XX_CMD_CFG register plays a role of a lock, to
> > > protect the controller from firmware access, which means the firmware
> > > cannot reach the controller if the driver set the bit. Add prepare/
> > > unprepare methods for the controller, we'll hold the lock in prepare
> > > method and release it in unprepare method, which will solve the race
> > > issue.
> >
> > I'm trying to understand the need for this change. What's wrong with
> > performing the lock/unlock procedure in hisi_sfc_v3xx_exec_op()? You can
> > probably do something like:
> >
> > hisi_sfc_v3xx_lock();
> > ret = hisi_sfc_v3xx_generic_exec_op(host, op, chip_select);
> > hisi_sfc_v3xx_unlock();
> > return ret;
> >
> > What's the benefit of making upper layers do this? Acquiring the lock is
> > a simple register write, so it should be relatively fast. Unless there
> > is a lot of contention on the lock between the firmware and kernel, I
> > would expect the performance impact to be minimal. Maybe you can run
> > some benchmarks and see if there is a real difference.
> >
> > > Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
> > > ---
> > > drivers/spi/spi-hisi-sfc-v3xx.c | 41 ++++++++++++++++++++++++++++++++++++++++-
> > > 1 file changed, 40 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/spi/spi-hisi-sfc-v3xx.c b/drivers/spi/spi-hisi-sfc-v3xx.c
> > > index e3b5725..13c161c 100644
> > > --- a/drivers/spi/spi-hisi-sfc-v3xx.c
> > > +++ b/drivers/spi/spi-hisi-sfc-v3xx.c
> > > @@ -18,6 +18,7 @@
> > > #define HISI_SFC_V3XX_VERSION (0x1f8)
> > >
> > > #define HISI_SFC_V3XX_CMD_CFG (0x300)
> > > +#define HISI_SFC_V3XX_CMD_CFG_LOCK BIT(20)
> > > #define HISI_SFC_V3XX_CMD_CFG_DUAL_IN_DUAL_OUT (1 << 17)
> > > #define HISI_SFC_V3XX_CMD_CFG_DUAL_IO (2 << 17)
> > > #define HISI_SFC_V3XX_CMD_CFG_FULL_DIO (3 << 17)
> > > @@ -41,6 +42,34 @@ struct hisi_sfc_v3xx_host {
> > > int max_cmd_dword;
> > > };
> > >
> > > +int hisi_sfc_v3xx_op_prepare(struct spi_mem *mem)
> > > +{
> > > + struct spi_device *spi = mem->spi;
> > > + struct hisi_sfc_v3xx_host *host;
> > > + u32 reg = HISI_SFC_V3XX_CMD_CFG_LOCK;
> > > +
> > > + host = spi_controller_get_devdata(spi->master);
> > > +
> > > + writel(reg, host->regbase + HISI_SFC_V3XX_CMD_CFG);
> > > +
> > > + reg = readl(host->regbase + HISI_SFC_V3XX_CMD_CFG);
> > > + if (!(reg & HISI_SFC_V3XX_CMD_CFG_LOCK))
> > > + return -EIO;
> >
> > IIUC, you are checking if you actually got the lock, and you won't get
> > the lock if the firmware is using the controller. So, is it a good idea
> > to give up so easily? Maybe we should do this in a loop at some
> > intervals, and only error out when we reach a number of failed attempts?
> >
> > > +
> > > + return 0;
> > > +}
> > > +
> > > +void hisi_sfc_v3xx_op_unprepare(struct spi_mem *mem)
> > > +{
> > > + struct spi_device *spi = mem->spi;
> > > + struct hisi_sfc_v3xx_host *host;
> > > +
> > > + host = spi_controller_get_devdata(spi->master);
> > > +
> > > + /* Release the lock and clear the command register. */
> > > + writel(0, host->regbase + HISI_SFC_V3XX_CMD_CFG);
> > > +}
> > > +
> > > #define HISI_SFC_V3XX_WAIT_TIMEOUT_US 1000000
> > > #define HISI_SFC_V3XX_WAIT_POLL_INTERVAL_US 10
> > >
> > > @@ -163,7 +192,15 @@ static int hisi_sfc_v3xx_generic_exec_op(struct hisi_sfc_v3xx_host *host,
> > > u8 chip_select)
> > > {
> > > int ret, len = op->data.nbytes;
> > > - u32 config = 0;
> > > + u32 config;
> > > +
> > > + /*
> > > + * The lock bit is in the command register. Clear the command
> > > + * field with lock bit held if it has been set in
> > > + * .prepare().
> > > + */
> > > + config = readl(host->regbase + HISI_SFC_V3XX_CMD_CFG);
> > > + config &= HISI_SFC_V3XX_CMD_CFG_LOCK;
> >
> > This will unlock the controller _before_ the driver issues
> > hisi_sfc_v3xx_read_databuf(). I'm not very familiar with the hardware,
> > but to me it seems like it can lead to a race. What if the firmware
> > issues a command that over-writes the databuf (I assume this is shared
> > between the two) before the driver gets a chance to copy that data to
> > the kernel buffer?
>
> Like Pratyush said, I don't see why you need to expose new
> prepare/unprepare steps. Looks like something entirely controller
> specific.
Sorry, this comment is misplaced, just like my understanding of the
problem :-).
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next prev parent reply other threads:[~2020-05-26 9:30 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-21 11:23 [RFC PATCH 0/3] Add prepare/unprepare method in spi_controller_mem_ops Yicong Yang
2020-05-21 11:23 ` [RFC PATCH 1/3] spi: spi-mem: add optional prepare/unprepare methods " Yicong Yang
2020-05-21 11:23 ` [RFC PATCH 2/3] mtd: spi-nor: Add prepare/unprepare support for spimem device Yicong Yang
2020-05-21 11:23 ` [RFC PATCH 3/3] spi: hisi-sfc-v3xx: Add prepare/unprepare methods to avoid race condition Yicong Yang
2020-05-25 16:14 ` Pratyush Yadav
2020-05-26 9:27 ` Boris Brezillon
2020-05-26 9:30 ` Boris Brezillon [this message]
2020-05-27 8:18 ` Yicong Yang
2020-05-27 9:33 ` Pratyush Yadav
2020-05-27 10:33 ` Yicong Yang
2020-05-26 9:43 ` Boris Brezillon
2020-05-27 8:55 ` Yicong Yang
2020-05-27 9:20 ` Boris Brezillon
2020-05-27 10:16 ` Yicong Yang
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