Subject: Re: [PATCH v2] net: dsa: drop some VLAs in switch.c

From: Andrew Lunn <[email-masked]>
Date: Mon, 7 May 2018 21:26:41 +0200

> >> +++ b/include/net/dsa.h
> >> @@ -256,6 +256,9 @@ struct dsa_switch {
> >>       /* Number of switch port queues */
> >>       unsigned int            num_tx_queues;
> >>
> >> +     unsigned long           *bitmap;
> >> +     unsigned long           _bitmap;
> >> +
> >>       /* Dynamically allocated ports, keep last */
> >>       size_t num_ports;
> >>       struct dsa_port ports[];
> >> diff --git a/net/dsa/dsa2.c b/net/dsa/dsa2.c
> >> index adf50fb..cebf35f0 100644
> >> --- a/net/dsa/dsa2.c
> >> +++ b/net/dsa/dsa2.c
> >> @@ -748,6 +748,20 @@ struct dsa_switch *dsa_switch_alloc(struct device *dev, size_t n)
> >>       if (!ds)
> >>               return NULL;
> >>
> >> +     /* We avoid allocating memory outside dsa_switch
> >> +      * if it is not needed.
> >> +      */
> >> +     if (n <= sizeof(ds->_bitmap) * 8) {
> >> +             ds->bitmap = &ds->_bitmap;
> >
> > Should not this be / BITS_PER_BYTE? If the sizeof(unsigned long) is <=
> > 8, then you don't need to allocate it, otherwise, you have to.

> This optimization will save us an allocation when number of ports is
> less than 32 or 64 (depending on arch).
> IMHO it's useful, if you consider that, right now, DSA works only with
> 12-ports switches.

Do you have a feeling for the savings? I don't see it being very
large, and given the extra code, it might actually be negative.

    Andrew