Re: [PATCH v2 2/2] HWRNG: thunderx: Add Cavium HWRNG driver for ThunderX SoC.

[David Daney]

On 08/23/2016 10:46 PM, Corentin LABBE wrote:
> Hello
>
> > +/* Read data from the RNG unit */
> > +static int cavium_rng_read(struct hwrng *rng, void *dat, size_t max, bool wait)
> > +{
> > +	struct cavium_rng *p = container_of(rng, struct cavium_rng, ops);
> > +	unsigned int size = max;
> > +
> > +	while (size >= 8) {
> > +		*((u64 *)dat) = readq(p->result);
> > +		size -= 8;
> > +		dat += 8;
> > +	}
>
> I think you could use readsq()
> This will increase throughput

If you look at the implementation of readsq(), you will see that it is a 
similar loop.  Since the overhead is primarily I/O latency from the RNG 
hardware, the throughput cannot really be changed with micro 
optimizations to this simple loop.

Also, on big-endian kernels, it appears that a loop of readq() and 
readsq() will give different results as readq will byte swap the result 
and readsq does not.  Since this is a RNG, the byte swapping is not 
important, but it is a difference.

Because of this, I think it should be acceptable to stick with the loop 
we currently have.

If the hwrng maintainers want to change the loop, to a readsq(), we 
might investigate this more.

Thanks,
David Daney