[RFC 1/9] regmap: Add USB support

[Mark Brown]

On Sun, Feb 16, 2020 at 06:21:09PM +0100, Noralf Trønnes wrote:

> Add support for regmap over USB for use with the Multifunction USB Device.
> Two endpoints IN/OUT are used. Up to 255 regmaps are supported on one USB
> interface. The register index width is always 32-bit, but the register
> value can be 8, 16 or 32 bits wide. LZ4 compression is supported on bulk
> transfers.

This looks like a custom thing for some particular devices rather than a
thing that will work for any USB device?  If that is the case then this
should have a more specific name.

> +++ b/drivers/base/regmap/regmap-usb.c
> @@ -0,0 +1,1026 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * Register map access API - USB support

Why is this GPL-2.0-or-later?  The rest of the code is just plain old
GPL-2.0.

Please also make the entire comment a C++ one so things look more
intentional.

> +
> +	ktime_t start; /* FIXME: Temporary debug/perf aid */

Add tracepoints, most of your debugging stuff looks like you want to use
tracepoints - you can easily work out time differences with them by
postprocessing the log, they get very fine grained timestamps.

> +	mutex_lock(&regmap_usb_interfaces_lock);
> +	list_for_each_entry(entry, &regmap_usb_interfaces, link)
> +		if (entry->interface == interface) {
> +			ruif = entry;
> +			ruif->refcount++;
> +			goto out_unlock;
> +		}
> +

You're missing some { } here.

> +static long mud_drm_throughput(ktime_t begin, ktime_t end, size_t len)
> +{
> +	long throughput;
> +
> +	throughput = ktime_us_delta(end, begin);
> +	throughput = throughput ? (len * 1000) / throughput : 0;
> +	throughput = throughput * 1000 / 1024;

Please write normal conditional statements to improve legibility.

> +static int regmap_usb_gather_write(void *context,
> +				   const void *reg, size_t reg_len,
> +				   const void *val, size_t val_len)
> +{
> +	return regmap_usb_transfer(context, false, reg, (void *)val, val_len);
> +}

Why are we casting away a const here?  If we really need to modify the
raw data that's being transmitted take a copy.

> +static int regmap_usb_write(void *context, const void *data, size_t count)
> +{
> +	struct regmap_usb_context *ctx = context;
> +	size_t val_len = count - sizeof(u32);
> +	void *val;
> +	int ret;
> +
> +	/* buffer needs to be properly aligned for DMA use */
> +	val = kmemdup(data + sizeof(u32), val_len, GFP_KERNEL);
> +	if (!val)
> +		return -ENOMEM;
> +
> +	ret = regmap_usb_gather_write(ctx, data, sizeof(u32), val, val_len);
> +	kfree(val);
> +
> +	return ret;
> +}

This looks like you just don't support a straight write operation, if
you need to do this emulation push it up the stack.

> +		regmap_usb_debugfs_init(map);
> +
> +	return map;
> +}
> +EXPORT_SYMBOL(__devm_regmap_init_usb);

No, this needs to be EXPORT_SYMBOL_GPL - the regmap core is and this
isn't going to be useful without those bits of the code anyway.
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