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    SubjectRe: [PATCHv2 2/2] hwrng: optee: fix wait use case
    On 24/07/20, Jorge Ramirez-Ortiz, Foundries wrote:
    > On 24/07/20, Sumit Garg wrote:
    > > On Thu, 23 Jul 2020 at 14:16, Jorge Ramirez-Ortiz <jorge@foundries.io> wrote:
    > > >
    > > > The current code waits for data to be available before attempting a
    > > > second read. However the second read would not be executed as the
    > > > while loop exits.
    > > >
    > > > This fix does not wait if all data has been read and reads a second
    > > > time if only partial data was retrieved on the first read.
    > > >
    > > > This fix also does not attempt to read if not data is requested.
    > >
    > > I am not sure how this is possible, can you elaborate?
    >
    > currently, if the user sets max 0, get_optee_rng_data will regardless
    > issuese a call to the secure world requesting 0 bytes from the RNG
    >
    > with this patch, this request is avoided.
    >
    > >
    > > >
    > > > Signed-off-by: Jorge Ramirez-Ortiz <jorge@foundries.io>
    > > > ---
    > > > v2: tidy up the while loop to avoid reading when no data is requested
    > > >
    > > > drivers/char/hw_random/optee-rng.c | 4 ++--
    > > > 1 file changed, 2 insertions(+), 2 deletions(-)
    > > >
    > > > diff --git a/drivers/char/hw_random/optee-rng.c b/drivers/char/hw_random/optee-rng.c
    > > > index 5bc4700c4dae..a99d82949981 100644
    > > > --- a/drivers/char/hw_random/optee-rng.c
    > > > +++ b/drivers/char/hw_random/optee-rng.c
    > > > @@ -122,14 +122,14 @@ static int optee_rng_read(struct hwrng *rng, void *buf, size_t max, bool wait)
    > > > if (max > MAX_ENTROPY_REQ_SZ)
    > > > max = MAX_ENTROPY_REQ_SZ;
    > > >
    > > > - while (read == 0) {
    > > > + while (read < max) {
    > > > rng_size = get_optee_rng_data(pvt_data, data, (max - read));
    > > >
    > > > data += rng_size;
    > > > read += rng_size;
    > > >
    > > > if (wait && pvt_data->data_rate) {
    > > > - if (timeout-- == 0)
    > > > + if ((timeout-- == 0) || (read == max))
    > >
    > > If read == max, would there be any sleep?
    >
    > no but I see no reason why there should be a wait since we already have
    > all the data that we need; the msleep is only required when we need to
    > wait for the RNG to generate entropy for the number of bytes we are
    > requesting. if we are requesting 0 bytes, the entropy is already
    > available. at leat this is what makes sense to me.
    >
    >

    any further comments?

    > >
    > > -Sumit
    > >
    > > > return read;
    > > > msleep((1000 * (max - read)) / pvt_data->data_rate);
    > > > } else {
    > > > --
    > > > 2.17.1
    > > >

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