linux-kernel - Re: [PATCH V3 11/14] coresight: sink: Add TRBE driver

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Message-ID: <20210212165734.GA2692426@xps15>
Date:   Fri, 12 Feb 2021 09:57:34 -0700
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, coresight@...ts.linaro.org,
        suzuki.poulose@....com, mike.leach@...aro.org,
        lcherian@...vell.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 11/14] coresight: sink: Add TRBE driver

[...]

> > 
> > 
> >> +	if (nr_pages < 2)
> >> +		return NULL;
> >> +
> >> +	buf = kzalloc_node(sizeof(*buf), GFP_KERNEL, trbe_alloc_node(event));
> >> +	if (IS_ERR(buf))
> >> +		return ERR_PTR(-ENOMEM);
> >> +
> >> +	pglist = kcalloc(nr_pages, sizeof(*pglist), GFP_KERNEL);
> >> +	if (IS_ERR(pglist)) {
> >> +		kfree(buf);
> >> +		return ERR_PTR(-ENOMEM);
> >> +	}
> >> +
> >> +	for (i = 0; i < nr_pages; i++)
> >> +		pglist[i] = virt_to_page(pages[i]);
> >> +
> >> +	buf->trbe_base = (unsigned long) vmap(pglist, nr_pages, VM_MAP, PAGE_KERNEL);
> >> +	if (IS_ERR((void *)buf->trbe_base)) {
> > 
> > Why not simply make buf->trbe_base a void * instead of having to do all this
> 
> There are many arithmetic and comparison operations involving trbe_base
> element. Hence it might be better to keep it as unsigned long, also to
> keeps it consistent with other pointers i.e trbe_write, trbe_limit.

That is a fair point.  Please add a comment to explain your design choice and
make sure the sparse checker is happy with all of it.

> 
> Snippet from $cat drivers/hwtracing/coresight/coresight-trbe.c | grep "trbe_base"
> There are just two places type casting trbe_base back to (void *).
> 
> 	memset((void *)buf->trbe_base + head, ETE_IGNORE_PACKET, len);
> 	return buf->trbe_base + offset;
> 	WARN_ON(buf->trbe_write < buf->trbe_base);
> 	set_trbe_base_pointer(buf->trbe_base);
> 	buf->trbe_base = (unsigned long)vmap(pglist, nr_pages, VM_MAP, PAGE_KERNEL);
> 	if (IS_ERR((void *)buf->trbe_base)) {
> 		return ERR_PTR(buf->trbe_base);
> 	buf->trbe_limit = buf->trbe_base + nr_pages * PAGE_SIZE;
> 	buf->trbe_write = buf->trbe_base;
> 	vunmap((void *)buf->trbe_base);
> 	base = get_trbe_base_pointer();
> 	buf->trbe_write = buf->trbe_base + PERF_IDX2OFF(handle->head, buf);
> 	if (buf->trbe_limit == buf->trbe_base) {
> 	buf->trbe_write = buf->trbe_base + PERF_IDX2OFF(handle->head, buf);
> 	if (buf->trbe_limit == buf->trbe_base) {
> 	offset = get_trbe_limit_pointer() - get_trbe_base_pointer();
> 	buf->trbe_write = buf->trbe_base + PERF_IDX2OFF(handle->head, buf);
> 	if (buf->trbe_limit == buf->trbe_base) {
> 	WARN_ON(buf->trbe_base != get_trbe_base_pointer());
> 		if (get_trbe_write_pointer() == get_trbe_base_pointer())
>   
> > casting?  And IS_ERR() doesn't work with vmap().
> 
> Sure, will drop IS_ERR() here.
> 

[...]


> > 
> >> +
> >> +static ssize_t dbm_show(struct device *dev, struct device_attribute *attr, char *buf)
> >> +{
> >> +	struct trbe_cpudata *cpudata = dev_get_drvdata(dev);
> >> +
> >> +	return sprintf(buf, "%d\n", cpudata->trbe_dbm);
> >> +}
> >> +static DEVICE_ATTR_RO(dbm);
> > 
> > What does "dbm" stand for?  Looking at the documentation for TRBIDR_EL1.F, I
> > don't see what "dbm" relates to.
> 
> I made it up to refer TRBIDR_EL1.F as "Dirty (and Access Flag) Bit Management".
> Could change it as "afdbm" to be more specific or if it is preferred.
> 

I don't see "afdbm" being a better solution - why not simply "flag"?