Re: [RFC PATCH 3/3] spi: hisi-sfc-v3xx: Add prepare/unprepare methods to avoid race condition

[Boris Brezillon <boris.brezillon@collabora.com>]

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.

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