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Date:   Fri, 22 Jul 2022 01:09:00 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        ZiyangZhang <ZiyangZhang@linux.alibaba.com>
Subject: Re: [PATCH V2 2/2] ublk_drv: make sure that correct flags(features)
 returned to userspace
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References: <20220722050930.611232-1-ming.lei@redhat.com>
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On Fri, Jul 22, 2022 at 03:30:17PM +0800, Ming Lei wrote:
> > > +enum ublk_flag_bits {
> > > +	__UBLK_F_SUPPORT_ZERO_COPY,
> > > +	__UBLK_F_URING_CMD_COMP_IN_TASK,
> > > +	__UBLK_F_NR_BITS,
> > > +};
> > 
> > Please make these #defines so that userspace can detect if they
> > exist in a header using #ifdef.
> 
> userspace is supposed to only use UBLK_F_* instead of __UBLK_F_*, one
> benefit of using enum is that UBLK_F_NR_BITS can be figured out
> automatically, otherwise how can we figure out the max bits?

Oh, indeed.  But I don't think the automatic max index is really
that useful.  Just use a manually maintained in-kernel UBLK_F_ALL,
that is just as little overhead to maintain as the extra enum.
And if you forget to add a flag that code will never see it as it
gets cleared before the device is added.