From: Vinod Subject: Re: [PATCH 1/2] hwrng: msm: add a spinlock and support for blocking reads Date: Fri, 22 Jun 2018 09:47:12 +0530 Message-ID: <20180622041712.GE27187@vkoul-mobl> References: <1529594276-12210-1-git-send-email-timur@codeaurora.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: timur@kernel.org, linux-arm-msm@vger.kernel.org, swboyd@chromium.org, linux-crypto@vger.kernel.org, Matt Mackall , linux-arm-kernel@lists.infradead.org To: Timur Tabi Return-path: Content-Disposition: inline In-Reply-To: <1529594276-12210-1-git-send-email-timur@codeaurora.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=m.gmane.org@lists.infradead.org List-Id: linux-crypto.vger.kernel.org On 21-06-18, 10:17, Timur Tabi wrote: > The hwrng.read callback includes a boolean parameter called 'wait' > which indicates whether the function should block and wait for > more data. > > When 'wait' is true, the driver spins on the DATA_AVAIL bit or until > a reasonable timeout. The timeout can occur if there is a heavy load > on reading the PRNG. > > The same code also needs a spinlock to protect against race conditions. > > If multiple cores hammer on the PRNG, it's possible for a race > condition to occur between reading the status register and > reading the data register. Add a spinlock to protect against > that. > > 1. Core 1 reads status register, shows data is available. > 2. Core 2 also reads status register, same result > 3. Core 2 reads data register, depleting all entropy > 4. Core 1 reads data register, which returns 0 > > Signed-off-by: Timur Tabi > --- > drivers/char/hw_random/msm-rng.c | 57 +++++++++++++++++++++++++++++++++++----- > 1 file changed, 50 insertions(+), 7 deletions(-) > > diff --git a/drivers/char/hw_random/msm-rng.c b/drivers/char/hw_random/msm-rng.c > index 841fee845ec9..44580588b938 100644 > --- a/drivers/char/hw_random/msm-rng.c > +++ b/drivers/char/hw_random/msm-rng.c > @@ -15,9 +15,11 @@ > #include > #include > #include > +#include > #include > #include > #include > +#include > > /* Device specific register offsets */ > #define PRNG_DATA_OUT 0x0000 > @@ -35,10 +37,22 @@ > #define MAX_HW_FIFO_SIZE (MAX_HW_FIFO_DEPTH * 4) > #define WORD_SZ 4 > > +/* > + * Normally, this would be the maximum time it takes to refill the FIFO, > + * after a read. Under heavy load, tests show that this delay is either > + * below 50us or above 2200us. The higher value is probably what happens > + * when entropy is completely depleted. > + * > + * Since we don't want to wait 2ms in a spinlock, set the timeout to the > + * lower value. Under extreme situations, that timeout can extend to 100us. > + */ > +#define TIMEOUT 50 > + > struct msm_rng { > void __iomem *base; > struct clk *clk; > struct hwrng hwrng; > + spinlock_t lock; > }; > > #define to_msm_rng(p) container_of(p, struct msm_rng, hwrng) > @@ -96,11 +110,39 @@ static int msm_rng_read(struct hwrng *hwrng, void *data, size_t max, bool wait) > > /* read random data from hardware */ > do { > - val = readl_relaxed(rng->base + PRNG_STATUS); > - if (!(val & PRNG_STATUS_DATA_AVAIL)) > - break; > + spin_lock(&rng->lock); > + > + /* > + * First check the status bit. If 'wait' is true, then wait > + * up to TIMEOUT microseconds for data to be available. > + */ > + if (wait) { > + int ret; > + > + ret = readl_poll_timeout_atomic(rng->base + PRNG_STATUS, > + val, val & PRNG_STATUS_DATA_AVAIL, 0, TIMEOUT); > + if (ret) { > + /* Timed out */ > + spin_unlock(&rng->lock); > + break; > + } > + } else { > + val = readl_relaxed(rng->base + PRNG_STATUS); > + if (!(val & PRNG_STATUS_DATA_AVAIL)) { > + spin_unlock(&rng->lock); > + break; > + } > + } > > val = readl_relaxed(rng->base + PRNG_DATA_OUT); > + spin_unlock(&rng->lock); > + > + /* > + * Zero is technically a valid random number, but it's also > + * the value returned if the PRNG is not enabled properly. > + * To avoid accidentally returning all zeros, treat it as > + * invalid and just return what we've already read. > + */ > if (!val) > break; > > @@ -148,10 +190,11 @@ static int msm_rng_probe(struct platform_device *pdev) > if (IS_ERR(rng->clk)) > return PTR_ERR(rng->clk); > > - rng->hwrng.name = KBUILD_MODNAME, > - rng->hwrng.init = msm_rng_init, > - rng->hwrng.cleanup = msm_rng_cleanup, > - rng->hwrng.read = msm_rng_read, > + rng->hwrng.name = KBUILD_MODNAME; > + rng->hwrng.init = msm_rng_init; > + rng->hwrng.cleanup = msm_rng_cleanup; > + rng->hwrng.read = msm_rng_read; this should be a separate patch -- ~Vinod