linux-kernel - Re: [RFC][PATCH 1/5 v2] dma-buf: Add dma-buf heaps framework

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Date:   Tue, 19 Mar 2019 10:24:04 -0500
From:   "Andrew F. Davis" <afd@...com>
To:     Brian Starkey <Brian.Starkey@....com>,
        John Stultz <john.stultz@...aro.org>
CC:     lkml <linux-kernel@...r.kernel.org>,
        Laura Abbott <labbott@...hat.com>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Liam Mark <lmark@...eaurora.org>,
        Chenbo Feng <fengc@...gle.com>,
        Alistair Strachan <astrachan@...gle.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        nd <nd@....com>
Subject: Re: [RFC][PATCH 1/5 v2] dma-buf: Add dma-buf heaps framework

On 3/19/19 7:08 AM, Brian Starkey wrote:
> Hi John,
> 
> On Tue, Mar 05, 2019 at 12:54:29PM -0800, John Stultz wrote:
>> From: "Andrew F. Davis" <afd@...com>
> 
> [snip]
> 
>> +
>> +#define NUM_HEAP_MINORS 128
>> +static DEFINE_IDR(dma_heap_idr);
>> +static DEFINE_MUTEX(minor_lock); /* Protect idr accesses */
> 
> I saw that Matthew Wilcox is trying to nuke idr:
> https://patchwork.freedesktop.org/series/57073/
> 
> Perhaps a different data structure could be considered? (I don't have
> an informed opinion on which).
> 

Looks like XArray is the suggested replacement. Should be easy enough,
the minor number would just index to our heap struct directly, I'll give
it a shot and see.

>> +
>> +dev_t dma_heap_devt;
>> +struct class *dma_heap_class;
>> +struct list_head dma_heap_list;
>> +struct dentry *dma_heap_debug_root;
>> +
>> +static int dma_heap_buffer_alloc(struct dma_heap *heap, size_t len,
>> +				 unsigned int flags)
>> +{
>> +	len = PAGE_ALIGN(len);
>> +	if (!len)
>> +		return -EINVAL;
> 
> I think aligning len to pages only makes sense if heaps are going to
> allocate aligned to pages too. Perhaps that's an implicit assumption?
> If so, lets document it.
> 
> Why not let the heaps take care of aligning len however they want
> though?
> 

This is how I originally had it, but I think we couldn't find any case
where you would want an the start or end of a buffer to not fall on a
page boundary here. It would only lead to problems. As you say though,
nothing keeping us from moving that into the heaps themselves.

> ...
> 
>> +
>> +int dma_heap_add(struct dma_heap *heap)
>> +{
>> +	struct device *dev_ret;
>> +	int ret;
>> +
>> +	if (!heap->name || !strcmp(heap->name, "")) {
>> +		pr_err("dma_heap: Cannot add heap without a name\n");
>> +		return -EINVAL;
>> +	}
>> +
>> +	if (!heap->ops || !heap->ops->allocate) {
>> +		pr_err("dma_heap: Cannot add heap with invalid ops struct\n");
>> +		return -EINVAL;
>> +	}
>> +
>> +	/* Find unused minor number */
>> +	mutex_lock(&minor_lock);
>> +	ret = idr_alloc(&dma_heap_idr, heap, 0, NUM_HEAP_MINORS, GFP_KERNEL);
>> +	mutex_unlock(&minor_lock);
>> +	if (ret < 0) {
>> +		pr_err("dma_heap: Unable to get minor number for heap\n");
>> +		return ret;
>> +	}
>> +	heap->minor = ret;
>> +
>> +	/* Create device */
>> +	heap->heap_devt = MKDEV(MAJOR(dma_heap_devt), heap->minor);
>> +	dev_ret = device_create(dma_heap_class,
>> +				NULL,
>> +				heap->heap_devt,
>> +				NULL,
>> +				heap->name);
>> +	if (IS_ERR(dev_ret)) {
>> +		pr_err("dma_heap: Unable to create char device\n");
>> +		return PTR_ERR(dev_ret);
>> +	}
>> +
>> +	/* Add device */
>> +	cdev_init(&heap->heap_cdev, &dma_heap_fops);
>> +	ret = cdev_add(&heap->heap_cdev, dma_heap_devt, NUM_HEAP_MINORS);
> 
> Shouldn't this be s/dma_heap_devt/heap->heap_devt/ and a count of 1?
> 

Hmm, strange this ever worked before..

> Also would it be better to have cdev_add/device_create the other way
> around? First create the char device, then once it's all set up
> register it with sysfs.
> 

Yes that does seem to be more common, lets flip it.

>> +	if (ret < 0) {
>> +		device_destroy(dma_heap_class, heap->heap_devt);
>> +		pr_err("dma_heap: Unable to add char device\n");
>> +		return ret;
>> +	}
>> +
>> +	return 0;
>> +}
>> +EXPORT_SYMBOL(dma_heap_add);
> 
> Until we've figured out how modules are going to work, I still think
> it would be a good idea to not export this.
> 

Agree.

Andrew

> Cheers,
> -Brian
>