Re: [PATCH v2 09/14] device core: Add ability to handle multiple dma offsets

[Nicolas Saenz Julienne]

Hi Jim,
one thing comes to mind, there is a small test suite in drivers/of/unittest.c
(specifically of_unittest_pci_dma_ranges()) you could extend it to include your
use cases.

On Tue, 2020-05-26 at 15:12 -0400, Jim Quinlan wrote:
> The new field in struct device 'dma_pfn_offset_map' is used to facilitate
> the use of multiple pfn offsets between cpu addrs and dma addrs.  It is
> similar to 'dma_pfn_offset' except that the offset chosen depends on the
> cpu or dma address involved.
> 
> Signed-off-by: Jim Quinlan <james.quinlan@xxxxxxxxxxxx>
> ---
>  drivers/of/address.c        | 65 +++++++++++++++++++++++++++++++++++--
>  drivers/usb/core/message.c  |  3 ++
>  drivers/usb/core/usb.c      |  3 ++
>  include/linux/device.h      | 10 +++++-
>  include/linux/dma-direct.h  | 10 ++++--
>  include/linux/dma-mapping.h | 46 ++++++++++++++++++++++++++
>  kernel/dma/Kconfig          | 13 ++++++++
>  7 files changed, 144 insertions(+), 6 deletions(-)
> 

[...]

> @@ -977,10 +1020,19 @@ int of_dma_get_range(struct device *dev, struct
> device_node *np, u64 *dma_addr,
>  		pr_debug("dma_addr(%llx) cpu_addr(%llx) size(%llx)\n",
>  			 range.bus_addr, range.cpu_addr, range.size);
>  
> +		num_ranges++;
>  		if (dma_offset && range.cpu_addr - range.bus_addr != dma_offset)
> {
> -			pr_warn("Can't handle multiple dma-ranges with different
> offsets on node(%pOF)\n", node);
> -			/* Don't error out as we'd break some existing DTs */
> -			continue;
> +			if (!IS_ENABLED(CONFIG_DMA_PFN_OFFSET_MAP)) {
> +				pr_warn("Can't handle multiple dma-ranges with
> different offsets on node(%pOF)\n", node);
> +				pr_warn("Perhaps set DMA_PFN_OFFSET_MAP=y?\n");
> +				/*
> +				 * Don't error out as we'd break some existing
> +				 * DTs that are using configs w/o
> +				 * CONFIG_DMA_PFN_OFFSET_MAP set.
> +				 */
> +				continue;

dev->bus_dma_limit is set in of_dma_configure(), this function's caller, based
on dma_start's value (set after this continue). So you'd be effectively setting
the dev->bus_dma_limit to whatever we get from the first dma-range.

This can be troublesome depending on how the dma-ranges are setup, for example
if the first dma-range doesn't include the CMA area, in arm64 generally set as
high as possible in ZONE_DMA32, that would render it useless for
dma/{direct/swiotlb}. Again depending on the bus_dma_limit value, if smaller
than ZONE_DMA you'd be unable to allocate any DMA memory.

IMO, a solution to this calls for a revamp of dma-direct's dma_capable(): match
the target DMA memory area with each dma-range we have to see if it fits.

> +			}
> +			dma_multi_pfn_offset = true;
>  		}
>  		dma_offset = range.cpu_addr - range.bus_addr;
>  
> @@ -991,6 +1043,13 @@ int of_dma_get_range(struct device *dev, struct
> device_node *np, u64 *dma_addr,
>  			dma_end = range.bus_addr + range.size;
>  	}
>  
> +	if (dma_multi_pfn_offset) {
> +		dma_offset = 0;
> +		ret = attach_dma_pfn_offset_map(dev, node, num_ranges);
> +		if (ret)
> +			return ret;
> +	}
> +
>  	if (dma_start >= dma_end) {
>  		ret = -EINVAL;
>  		pr_debug("Invalid DMA ranges configuration on node(%pOF)\n",
> diff --git a/drivers/usb/core/message.c b/drivers/usb/core/message.c
> index 6197938dcc2d..aaa3e58f5eb4 100644
> --- a/drivers/usb/core/message.c
> +++ b/drivers/usb/core/message.c
> @@ -1960,6 +1960,9 @@ int usb_set_configuration(struct usb_device *dev, int
> configuration)
>  		 */
>  		intf->dev.dma_mask = dev->dev.dma_mask;
>  		intf->dev.dma_pfn_offset = dev->dev.dma_pfn_offset;
> +#ifdef CONFIG_DMA_PFN_OFFSET_MAP
> +		intf->dev.dma_pfn_offset_map = dev->dev.dma_pfn_offset_map;
> +#endif

Thanks for looking at this, that said, I see more instances of drivers changing
dma_pfn_offset outside of the core code. Why not doing this there too?

Also, are we 100% sure that dev->dev.dma_pfn_offset isn't going to be freed
before we're done using intf->dev? Maybe it's safer to copy the ranges?

>  		INIT_WORK(&intf->reset_ws, __usb_queue_reset_device);
>  		intf->minor = -1;
>  		device_initialize(&intf->dev);
> diff --git a/drivers/usb/core/usb.c b/drivers/usb/core/usb.c
> index f16c26dc079d..d2ed4d90e56e 100644
> --- a/drivers/usb/core/usb.c
> +++ b/drivers/usb/core/usb.c
> @@ -612,6 +612,9 @@ struct usb_device *usb_alloc_dev(struct usb_device
> *parent,
>  	 */
>  	dev->dev.dma_mask = bus->sysdev->dma_mask;
>  	dev->dev.dma_pfn_offset = bus->sysdev->dma_pfn_offset;
> +#ifdef CONFIG_DMA_PFN_OFFSET_MAP
> +	dev->dev.dma_pfn_offset_map = bus->sysdev->dma_pfn_offset_map;
> +#endif
>  	set_dev_node(&dev->dev, dev_to_node(bus->sysdev));
>  	dev->state = USB_STATE_ATTACHED;
>  	dev->lpm_disable_count = 1;
> diff --git a/include/linux/device.h b/include/linux/device.h
> index ac8e37cd716a..67a240ad4fc5 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -493,6 +493,8 @@ struct dev_links_info {
>   * @bus_dma_limit: Limit of an upstream bridge or bus which imposes a smaller
>   *		DMA limit than the device itself supports.
>   * @dma_pfn_offset: offset of DMA memory range relatively of RAM
> + * @dma_pfn_offset_map:	Like dma_pfn_offset but used when there are
> multiple
> + *		pfn offsets for multiple dma-ranges.
>   * @dma_parms:	A low level driver may set these to teach IOMMU code
> about
>   * 		segment limitations.
>   * @dma_pools:	Dma pools (if dma'ble device).
> @@ -578,7 +580,13 @@ struct device {
>  					     allocations such descriptors. */
>  	u64		bus_dma_limit;	/* upstream dma constraint */
>  	unsigned long	dma_pfn_offset;
> -
> +#ifdef CONFIG_DMA_PFN_OFFSET_MAP
> +	const struct dma_pfn_offset_region *dma_pfn_offset_map;
> +					/* Like dma_pfn_offset, but for
> +					 * the unlikely case of multiple
> +					 * offsets. If non-null, dma_pfn_offset
> +					 * will be set to 0. */
> +#endif

I'm still sad this doesn't fully replace dma_pfn_offset & bus_dma_limit. I feel
the extra logic involved in incorporating this as default isn't going to be
noticeable as far as performance is concerned to single dma-range users, and
it'd make for a nicer DMA code. Also you'd force everyone to test their changes
on the multi dma-ranges code path, as opposed to having this disabled 99.9% of
the time (hence broken every so often).

Note that I sympathize with the amount of work involved on improving that, so
better wait to hear what more knowledgeable people have to say about this :)

Regards,
Nicolas

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