Message-Id: <d97de592-d3c6-4683-ab36-4ea2e8bd27b7@www.fastmail.com>
Date:   Wed, 11 Dec 2019 14:18:35 +1030
From:   "Andrew Jeffery" <andrew@...id.au>
To:     "Eddie James" <eajames@...ux.ibm.com>, linux-kernel@...r.kernel.org
Cc:     devicetree@...r.kernel.org, "Jason Cooper" <jason@...edaemon.net>,
        linux-aspeed@...ts.ozlabs.org, "Marc Zyngier" <maz@...nel.org>,
        "Rob Herring" <robh+dt@...nel.org>, tglx@...utronix.de,
        mark.rutland@....com, "Joel Stanley" <joel@....id.au>
Subject: Re: [PATCH v2 07/12] drivers/soc: xdma: Add user interface



On Fri, 6 Dec 2019, at 03:45, Eddie James wrote:
> This commits adds a miscdevice to provide a user interface to the XDMA
> engine. The interface provides the write operation to start DMA
> operations. The DMA parameters are passed as the data to the write call.
> The actual data to transfer is NOT passed through write. Note that both
> directions of DMA operation are accomplished through the write command;
> BMC to host and host to BMC.
> 
> The XDMA engine is restricted to only accessing the reserved memory
> space on the AST2500, typically used by the VGA. For this reason, the
> VGA memory space is pooled and allocated with genalloc. Users calling
> mmap allocate pages from this pool for their usage. The space allocated
> by a client will be the space used in the DMA operation. For an
> "upstream" (BMC to host) operation, the data in the client's area will
> be transferred to the host. For a "downstream" (host to BMC) operation,
> the host data will be placed in the client's memory area.

Given the comments on earlier patches we should reconsider descriptions
of the VGA area in this paragraph.

> 
> Poll is also provided in order to determine when the DMA operation is
> complete for non-blocking IO.
> 
> Signed-off-by: Eddie James <eajames@...ux.ibm.com>
> ---
> Changes since v1:
>  - Add file_lock comment
>  - Bring user reset up to date with new reset method
> 
>  drivers/soc/aspeed/aspeed-xdma.c | 224 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 224 insertions(+)
> 
> diff --git a/drivers/soc/aspeed/aspeed-xdma.c b/drivers/soc/aspeed/aspeed-xdma.c
> index a9b3eeb..d4b96a7 100644
> --- a/drivers/soc/aspeed/aspeed-xdma.c
> +++ b/drivers/soc/aspeed/aspeed-xdma.c
> @@ -13,6 +13,7 @@
>  #include <linux/io.h>
>  #include <linux/jiffies.h>
>  #include <linux/mfd/syscon.h>
> +#include <linux/miscdevice.h>
>  #include <linux/module.h>
>  #include <linux/mutex.h>
>  #include <linux/of_device.h>
> @@ -206,6 +207,8 @@ struct aspeed_xdma {
>  	struct clk *clock;
>  	struct reset_control *reset;
>  
> +	/* file_lock serializes reads of current_client */
> +	struct mutex file_lock;
>  	struct aspeed_xdma_client *current_client;
>  
>  	/* start_lock protects cmd_idx, cmdq, and the state of the engine */
> @@ -227,6 +230,8 @@ struct aspeed_xdma {
>  	dma_addr_t cmdq_vga_phys;
>  	void *cmdq_vga_virt;
>  	struct gen_pool *vga_pool;
> +
> +	struct miscdevice misc;
>  };
>  
>  struct aspeed_xdma_client {
> @@ -517,6 +522,207 @@ static irqreturn_t aspeed_xdma_pcie_irq(int irq, 
> void *arg)
>  	return IRQ_HANDLED;
>  }
>  
> +static ssize_t aspeed_xdma_write(struct file *file, const char __user *buf,
> +				 size_t len, loff_t *offset)
> +{
> +	int rc;
> +	struct aspeed_xdma_op op;
> +	struct aspeed_xdma_client *client = file->private_data;
> +	struct aspeed_xdma *ctx = client->ctx;
> +	u32 offs = client->phys ? (client->phys - ctx->vga_phys) :
> +		XDMA_CMDQ_SIZE;
> +
> +	if (len != sizeof(op))
> +		return -EINVAL;
> +
> +	rc = copy_from_user(&op, buf, len);
> +	if (rc)
> +		return rc;
> +
> +	if (op.direction == ASPEED_XDMA_DIRECTION_RESET) {
> +		unsigned long flags;
> +
> +		spin_lock_irqsave(&ctx->reset_lock, flags);
> +		if (ctx->in_reset) {
> +			spin_unlock_irqrestore(&ctx->reset_lock, flags);
> +			return len;
> +		}
> +
> +		ctx->in_reset = true;
> +		spin_unlock_irqrestore(&ctx->reset_lock, flags);
> +
> +		mutex_lock(&ctx->start_lock);
> +
> +		aspeed_xdma_reset(ctx);
> +
> +		mutex_unlock(&ctx->start_lock);
> +
> +		return len;
> +	} else if (op.direction > ASPEED_XDMA_DIRECTION_RESET) {
> +		return -EINVAL;
> +	}
> +
> +	if (op.len > ctx->vga_size - offs)
> +		return -EINVAL;

I'm wondering if we can rearrange the code to move the sanity checks to the
top of the function, so this and the `op.direction >
ASPEED_XDMA_DIRECTION_RESET` case.

The check above should fail for the reset case as well, I expect op.len should
be set to zero in that case. But I still think that jamming the reset command
into a "direction" concept feels broken, so as mentioned on an earlier patch
I'd prefer we move that distraction out to a separate patch.

> +
> +	if (file->f_flags & O_NONBLOCK) {
> +		if (!mutex_trylock(&ctx->file_lock))
> +			return -EAGAIN;
> +
> +		if (ctx->current_client) {
> +			mutex_unlock(&ctx->file_lock);
> +			return -EAGAIN;

I think EBUSY is better here.

> +		}
> +	} else {
> +		mutex_lock(&ctx->file_lock);
> +
> +		rc = wait_event_interruptible(ctx->wait, !ctx->current_client);
> +		if (rc) {
> +			mutex_unlock(&ctx->file_lock);
> +			return -EINTR;
> +		}
> +	}
> +
> +	aspeed_xdma_start(ctx, &op, ctx->vga_phys + offs, client);
> +
> +	mutex_unlock(&ctx->file_lock);

You've used file_lock here to protect aspeed_xdma_start() but start_lock
above to protect aspeed_xdma_reset(), so it seems one client can disrupt
another by resetting the engine while a DMA is in progress?

> +
> +	if (!(file->f_flags & O_NONBLOCK)) {
> +		rc = wait_event_interruptible(ctx->wait, !client->in_progress);
> +		if (rc)
> +			return -EINTR;
> +
> +		if (client->error)
> +			return -EIO;
> +	}
> +
> +	return len;
> +}
> +
> +static __poll_t aspeed_xdma_poll(struct file *file,
> +				 struct poll_table_struct *wait)
> +{
> +	__poll_t mask = 0;
> +	__poll_t req = poll_requested_events(wait);
> +	struct aspeed_xdma_client *client = file->private_data;
> +	struct aspeed_xdma *ctx = client->ctx;
> +
> +	if (req & (EPOLLIN | EPOLLRDNORM)) {
> +		if (client->in_progress)
> +			poll_wait(file, &ctx->wait, wait);
> +
> +		if (!client->in_progress) {
> +			if (client->error)
> +				mask |= EPOLLERR;
> +			else
> +				mask |= EPOLLIN | EPOLLRDNORM;
> +		}
> +	}
> +
> +	if (req & (EPOLLOUT | EPOLLWRNORM)) {
> +		if (ctx->current_client)
> +			poll_wait(file, &ctx->wait, wait);
> +
> +		if (!ctx->current_client)
> +			mask |= EPOLLOUT | EPOLLWRNORM;
> +	}
> +
> +	return mask;
> +}
> +
> +static void aspeed_xdma_vma_close(struct vm_area_struct *vma)
> +{
> +	struct aspeed_xdma_client *client = vma->vm_private_data;
> +
> +	gen_pool_free(client->ctx->vga_pool, (unsigned long)client->virt,
> +		      client->size);

What assurance do we have that a DMA isn't in progress? With non-blocking
IO we could easily start one then close the file descriptor, which would cause
havoc if the physical range is reused by a subsequent mapping.

> +
> +	client->virt = NULL;
> +	client->phys = 0;
> +	client->size = 0;
> +}
> +
> +static const struct vm_operations_struct aspeed_xdma_vm_ops = {
> +	.close =	aspeed_xdma_vma_close,
> +};
> +
> +static int aspeed_xdma_mmap(struct file *file, struct vm_area_struct *vma)
> +{
> +	int rc;
> +	struct aspeed_xdma_client *client = file->private_data;
> +	struct aspeed_xdma *ctx = client->ctx;
> +
> +	/* restrict file to one mapping */
> +	if (client->size)
> +		return -ENOMEM;
> +
> +	client->size = vma->vm_end - vma->vm_start;
> +	client->virt = gen_pool_dma_alloc(ctx->vga_pool, client->size,
> +					  &client->phys);
> +	if (!client->virt) {
> +		client->phys = 0;
> +		client->size = 0;
> +		return -ENOMEM;
> +	}
> +
> +	vma->vm_pgoff = (client->phys - ctx->vga_phys) >> PAGE_SHIFT;
> +	vma->vm_ops = &aspeed_xdma_vm_ops;
> +	vma->vm_private_data = client;
> +	vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> +
> +	rc = io_remap_pfn_range(vma, vma->vm_start, client->phys >> PAGE_SHIFT,
> +				client->size, vma->vm_page_prot);
> +	if (rc) {
> +		gen_pool_free(ctx->vga_pool, (unsigned long)client->virt,
> +			      client->size);
> +
> +		client->virt = NULL;
> +		client->phys = 0;
> +		client->size = 0;
> +		return rc;
> +	}
> +
> +	dev_dbg(ctx->dev, "mmap: v[%08lx] to p[%08x], s[%08x]\n",
> +		vma->vm_start, (u32)client->phys, client->size);
> +
> +	return 0;
> +}
> +
> +static int aspeed_xdma_open(struct inode *inode, struct file *file)
> +{
> +	struct miscdevice *misc = file->private_data;
> +	struct aspeed_xdma *ctx = container_of(misc, struct aspeed_xdma, misc);
> +	struct aspeed_xdma_client *client = kzalloc(sizeof(*client),
> +						    GFP_KERNEL);
> +
> +	if (!client)
> +		return -ENOMEM;
> +
> +	client->ctx = ctx;
> +	file->private_data = client;
> +	return 0;
> +}
> +
> +static int aspeed_xdma_release(struct inode *inode, struct file *file)
> +{
> +	struct aspeed_xdma_client *client = file->private_data;
> +
> +	if (client->ctx->current_client == client)
> +		client->ctx->current_client = NULL;

Shouldn't we also cancel the DMA op? This seems like a DoS risk: set up
a non-blocking, large downstream transfer then close the client. Also risks
scribbling on memory we no-longer own given we don't cancel/wait for
completion in vm close callback?

> +
> +	kfree(client);
> +	return 0;
> +}
> +
> +static const struct file_operations aspeed_xdma_fops = {
> +	.owner			= THIS_MODULE,
> +	.write			= aspeed_xdma_write,
> +	.poll			= aspeed_xdma_poll,
> +	.mmap			= aspeed_xdma_mmap,
> +	.open			= aspeed_xdma_open,
> +	.release		= aspeed_xdma_release,
> +};
> +
>  static int aspeed_xdma_probe(struct platform_device *pdev)
>  {
>  	int irq;
> @@ -539,6 +745,7 @@ static int aspeed_xdma_probe(struct platform_device *pdev)
>  	ctx->chip = md;
>  	ctx->dev = dev;
>  	platform_set_drvdata(pdev, ctx);
> +	mutex_init(&ctx->file_lock);
>  	mutex_init(&ctx->start_lock);
>  	INIT_WORK(&ctx->reset_work, aspeed_xdma_reset_work);
>  	spin_lock_init(&ctx->reset_lock);
> @@ -678,6 +885,22 @@ static int aspeed_xdma_probe(struct platform_device *pdev)
>  
>  	aspeed_xdma_init_eng(ctx);
>  
> +	ctx->misc.minor = MISC_DYNAMIC_MINOR;
> +	ctx->misc.fops = &aspeed_xdma_fops;
> +	ctx->misc.name = "aspeed-xdma";
> +	ctx->misc.parent = dev;
> +	rc = misc_register(&ctx->misc);
> +	if (rc) {
> +		dev_err(dev, "Failed to register xdma miscdevice.\n");
> +
> +		gen_pool_free(ctx->vga_pool, (unsigned long)ctx->cmdq_vga_virt,
> +			      XDMA_CMDQ_SIZE);
> +
> +		reset_control_assert(ctx->reset);
> +		clk_disable_unprepare(ctx->clock);
> +		return rc;
> +	}
> +
>  	/*
>  	 * This interrupt could fire immediately so only request it once the
>  	 * engine and driver are initialized.
> @@ -699,6 +922,7 @@ static int aspeed_xdma_remove(struct platform_device *pdev)
>  {
>  	struct aspeed_xdma *ctx = platform_get_drvdata(pdev);
>  
> +	misc_deregister(&ctx->misc);
>  	gen_pool_free(ctx->vga_pool, (unsigned long)ctx->cmdq_vga_virt,
>  		      XDMA_CMDQ_SIZE);
>  
> -- 
> 1.8.3.1
> 
>

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