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SubjectRe: [PATCH v2 09/14] device core: Add ability to handle multiple dma offsets
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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@broadcom.com>
> ---
> 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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