linux-kernel - Re: [PATCH 2/2] dmaengine: apple-sio: Add Apple SIO driver

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Date:   Thu, 3 Aug 2023 17:04:22 +0530
From:   Vinod Koul <vkoul@...nel.org>
To:     Martin Povišer <povik+lin@...ebit.org>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>, asahi@...ts.linux.dev,
        dmaengine@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] dmaengine: apple-sio: Add Apple SIO driver

On 03-08-23, 10:32, Martin Povišer wrote:

> >>> +static int sio_alloc_tag(struct sio_data *sio)
> >>> +{
> >>> +	struct sio_tagdata *tags = &sio->tags;
> >>> +	int tag, i;
> >>> +
> >>> +	/*
> >>> +	 * Because tag number 0 is special, the usable tag range
> >>> +	 * is 1...(SIO_NTAGS - 1). So, to pick the next usable tag,
> >>> +	 * we do modulo (SIO_NTAGS - 1) *then* plus one.
> >>> +	 */
> >>> +
> >>> +#define SIO_USABLE_TAGS (SIO_NTAGS - 1)
> >>> +	tag = (READ_ONCE(tags->last_tag) % SIO_USABLE_TAGS) + 1;
> >>> +
> >>> +	for (i = 0; i < SIO_USABLE_TAGS; i++) {
> >>> +		if (!test_and_set_bit(tag, &tags->allocated))
> >>> +			break;
> >>> +
> >>> +		tag = (tag % SIO_USABLE_TAGS) + 1;
> >>> +	}
> >>> +
> >>> +	WRITE_ONCE(tags->last_tag, tag);
> >>> +
> >>> +	if (i < SIO_USABLE_TAGS)
> >>> +		return tag;
> >>> +	else
> >>> +		return -EBUSY;
> >>> +#undef SIO_USABLE_TAGS
> >>> +}
> >> 
> >> can you use kernel mechanisms like ida to alloc and free the tags...
> > 
> > I can look into that.
> 
> Documentation says IDA is deprecated in favour of Xarray, both look
> like they serve to associate a pointer with an ID. I think neither
> structure beats a simple bitfield and a static array for the per-tag
> data. Agree?

yeah xarray am not too sure. I would still go with ida, we will see when
it is relly removed.

If you need a bitfield why not use bitmap apis.
I dont like drivers implementing the basic logic which kernel provides

-- 
~Vinod