On Mon, 25 May 2020 at 09:24, Nicolas TOROMANOFF <nicolas.toroman...@st.com> wrote: > > Hello, > > > -----Original Message----- > > From: Ard Biesheuvel <a...@kernel.org> > > Sent: Friday, May 22, 2020 6:12 PM> > > On Tue, 12 May 2020 at 16:13, Nicolas Toromanoff > > <nicolas.toroman...@st.com> wrote: > > > > > > Protect STM32 CRC device from concurrent accesses. > > > > > > As we create a spinlocked section that increase with buffer size, we > > > provide a module parameter to release the pressure by splitting > > > critical section in chunks. > > > > > > Size of each chunk is defined in burst_size module parameter. > > > By default burst_size=0, i.e. don't split incoming buffer. > > > > > > Signed-off-by: Nicolas Toromanoff <nicolas.toroman...@st.com> > > > > Would you mind explaining the usage model here? It looks like you are > > sharing a > > CRC hardware accelerator with a synchronous interface between different > > users > > by using spinlocks? You are aware that this will tie up the waiting CPUs > > completely during this time, right? So it would be much better to use a > > mutex > > here. Or perhaps it would make more sense to fall back to a s/w based CRC > > routine if the h/w is tied up working for another task? > > I know mutex are more acceptable here, but shash _update() and _init() may be > call > from any context, and so I cannot take a mutex. > And to protect my concurrent HW access I only though about spinlock. Due to > possible > constraint on CPUs, I add a burst_size option to force slitting long buffer > into smaller one, > and so decrease time we take the lock. > But I didn't though to fallback to software CRC. > > I'll do a patch on top. > In in the burst_update() function I'll use a spin_trylock_irqsave() and use > software CRC32 if HW is already in use. >
Right. I didn't even notice that you were keeping interrupts disabled the whole time when using the h/w block. That means that any serious use of this h/w block will make IRQ latency go through the roof. I recommend that you go back to the drawing board on this driver, rather than papering over the issues with a spin_trylock(). Perhaps it would be better to model it as a ahash (even though the h/w block itself is synchronous) and use a kthread to feed in the data.
