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Date:   Fri, 31 Aug 2018 15:36:53 +0200
From:   Sascha Hauer <s.hauer@...gutronix.de>
To:     Krzysztof Kozlowski <krzk@...nel.org>
Cc:     herbert@...dor.apana.org.au, davem@...emloft.net,
        robh+dt@...nel.org, mark.rutland@....com, shawnguo@...nel.org,
        kernel@...gutronix.de, Stefan Agner <stefan@...er.ch>,
        fabio.estevam@....com, linux-imx@....com, mturquette@...libre.com,
        sboyd@...nel.org, linux-crypto@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-clk@...r.kernel.org
Subject: Re: [PATCH 6/6] crypto: vf-crc - Add new driver for Freescale Vybrid
 CRC

On Fri, Aug 31, 2018 at 01:07:39PM +0200, Krzysztof Kozlowski wrote:
> On Fri, 31 Aug 2018 at 09:39, Sascha Hauer <s.hauer@...gutronix.de> wrote:
> >
> > Hi Krzysztof,
> >
> > Some comments inline.
> >
> > On Thu, Aug 30, 2018 at 07:15:39PM +0200, Krzysztof Kozlowski wrote:
> > > Add driver for using the Freescale/NXP Vybrid processor CRC block for
> > > CRC16 and CRC32 offloading.  The driver implements shash_alg and was
> > > tested using internal testmgr tests and libkcapi.
> > >
> > > Signed-off-by: Krzysztof Kozlowski <krzk@...nel.org>
> > > ---
> > >  MAINTAINERS               |   7 +
> > >  drivers/crypto/Kconfig    |  10 ++
> > >  drivers/crypto/Makefile   |   1 +
> > >  drivers/crypto/vf-crc.c   | 387 ++++++++++++++++++++++++++++++++++++++++++++++
> > >  include/linux/crc32poly.h |   7 +
> > >  5 files changed, 412 insertions(+)
> > >  create mode 100644 drivers/crypto/vf-crc.c
> > >
> > > diff --git a/MAINTAINERS b/MAINTAINERS
> > > index 0a340f680230..e84fa829a4e4 100644
> > > --- a/MAINTAINERS
> > > +++ b/MAINTAINERS
> > > @@ -15388,6 +15388,13 @@ S:   Maintained
> > >  F:   Documentation/fb/uvesafb.txt
> > >  F:   drivers/video/fbdev/uvesafb.*
> > >
> > > +VF500/VF610 HW CRC DRIVER
> > > +M:   Krzysztof Kozlowski <krzk@...nel.org>
> > > +L:   linux-crypto@...r.kernel.org
> > > +S:   Maintained
> > > +F:   drivers/crypto/vf-crc.c
> > > +F:   Documentation/devicetree/bindings/crypto/fsl-vf610-crc.txt
> > > +
> > >  VF610 NAND DRIVER
> > >  M:   Stefan Agner <stefan@...er.ch>
> > >  L:   linux-mtd@...ts.infradead.org
> > > diff --git a/drivers/crypto/Kconfig b/drivers/crypto/Kconfig
> > > index 20314d7a7b58..0ade940ac79c 100644
> > > --- a/drivers/crypto/Kconfig
> > > +++ b/drivers/crypto/Kconfig
> > > @@ -418,6 +418,16 @@ config CRYPTO_DEV_MXS_DCP
> > >         To compile this driver as a module, choose M here: the module
> > >         will be called mxs-dcp.
> > >
> > > +config CRYPTO_DEV_VF_CRC
> > > +     tristate "Support for Freescale/NXP Vybrid CRC HW accelerator"
> > > +     select CRYPTO_HASH
> > > +     help
> > > +       This option enables support for the CRC16/32 hardware accelerator
> > > +       on Freescale/NXP Vybrid VF500/VF610 SoCs.
> > > +
> > > +       To compile this driver as a module, choose M here: the module
> > > +       will be called vf-crc.
> > > +
> > >  config CRYPTO_DEV_EXYNOS_RNG
> > >       tristate "EXYNOS HW pseudo random number generator support"
> > >       depends on ARCH_EXYNOS || COMPILE_TEST
> > > diff --git a/drivers/crypto/Makefile b/drivers/crypto/Makefile
> > > index c23396f32c8a..418c08bdc19c 100644
> > > --- a/drivers/crypto/Makefile
> > > +++ b/drivers/crypto/Makefile
> > > @@ -41,6 +41,7 @@ obj-$(CONFIG_ARCH_STM32) += stm32/
> > >  obj-$(CONFIG_CRYPTO_DEV_SUN4I_SS) += sunxi-ss/
> > >  obj-$(CONFIG_CRYPTO_DEV_TALITOS) += talitos.o
> > >  obj-$(CONFIG_CRYPTO_DEV_UX500) += ux500/
> > > +obj-$(CONFIG_CRYPTO_DEV_VF_CRC) += vf-crc.o
> > >  obj-$(CONFIG_CRYPTO_DEV_VIRTIO) += virtio/
> > >  obj-$(CONFIG_CRYPTO_DEV_VMX) += vmx/
> > >  obj-$(CONFIG_CRYPTO_DEV_BCM_SPU) += bcm/
> > > +static int vf_crc_update_prepare(struct vf_crc_tfm_ctx *mctx,
> > > +                              struct vf_crc_desc_ctx *desc_ctx)
> > > +{
> > > +     struct vf_crc *crc = desc_ctx->crc;
> > > +     int ret;
> > > +
> > > +     ret = clk_prepare_enable(crc->clk);
> > > +     if (ret) {
> > > +             dev_err(crc->dev, "Failed to enable clock\n");
> > > +             return ret;
> > > +     }
> >
> > Generally have you measured the performance of this driver? Is it faster
> > than the software implementation?
> 
> I wanted to replace our in-house out-of-tree, hacky ioctl-based driver
> with something more upstreamable. I run few simple user-space
> performance tests and in fact SW implementation is faster. Around 5x
> faster for this version of driver. However it depends highly on size
> of message (buffer) because there is big overhead of libkcapi.

Well, I meant comparing the hardware vs. software implementation directly
in the kernel. Of course when a userspace API is involved the comparison
is not fair.

> 
> The typical SW implementation (with lookup tables) is just fetching of
> data from memory and computing. Usage of libkcapi is at least three
> library function calls on user-space side and a bunch of other code on
> kernel side.
> 
> There are two benefits:
> 1. CPU could be offloaded and do something in parallel. However for
> this I should probably implement asymmetric hash. Otherwise wastes
> cycles on reading from CRC registers... and of course on clk prepare
> and mutex handing.

The CPU can only do something in parallel when it's otherwise idle. In
your driver the CPU is 100% busy, so no time to do something else.

> 2. Theoretically it could lower energy consumption... as CPU would not
> be that busy. I found 3% lower power usage (0.18 A -> 0.175 A) but if
> you multiply it per time then total energy spent would be higher.
> 
> Does this driver makes sense in such case? In fact I have doubts...
> 
> It was nice exercise for me though. :)
> 
> >
> > Under certain circumstances a clk_prepare_enable might become expensive,
> > so it could happen that all this clk enabling/disabling takes longer
> > than the action you do in between. Using pm_runtime might help here.
> 
> I should convert them to just clk_enable/disable. The pm_runtime is
> also a huge framework and adds its own overhead. Using it just to
> toggle one clock is a lot.

There are probably more drivers in your system that make use of
pm_runtime, so no need to add it only for this one driver.

Sascha


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