Re: [PATCH] usb: early: xhci-dbc: Ensure correct memory size for early_ioremap()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, May 20, 2026 at 12:27:39PM +0000, Umang Jain wrote:
> Hi Greg,
> 
> On 2026-05-20 11:22, Greg Kroah-Hartman wrote:
> > On Wed, May 20, 2026 at 03:18:04PM +0530, Umang Jain wrote:
> >> early_ioremap() checks and fail, if the memory size exceeds the fixed
> >> boot-time mappings (dictated by NR_FIX_BTMAPS macro). We should ensure
> >> the correct maximum memory size is passed to early_ioremap() in the
> >> driver.
> >> 
> >> Without this check and page size being 4K(4096), enabling xhci-dbc
> >> on steamdeck seems to issue the warning:
> >> 
> >> steamdeck kernel: xhci_dbc:early_xdbc_parse_parameter: dbgp_num: 0
> >> steamdeck kernel: ------------[ cut here ]------------
> >> ay 19 13:42:57 steamdeck kernel: WARNING: CPU: 0 PID: 0 at mm/early_ioremap.c:139 __early_ioremap+0xae/0x180
> >> steamdeck kernel: Modules linked in:
> >> steamdeck kernel: CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted 6.18.25-valve1-gcab630e7af50 #13 PREEMPT(undef)  4b70872d9de6788b7f2d10dce46ada89e6cd177b
> >> steamdeck kernel: RIP: 0010:__early_ioremap+0xae/0x180
> >> steamdeck kernel: Code: 60 ba 3f a0 4c 89 ca 48 81 e3 00 f0 ff ff 48 81 e2 00 f0 ff ff 48 29 d3 48 89 14 24 48 89 da 48 c1 ea 0c 89 d5 83 fa 40 76 04 <0f> 0b eb a2 6b c0 c0 4d 89 ce 41 81 e6 ff 0f 00 00 44 8d b8 ff 05
> >> steamdeck kernel: RSP: 0000:ffffffff9fa03cb8 EFLAGS: 00010006 ORIG_RAX: 0000000000000000
> >> steamdeck kernel: RAX: 0000000000000000 RBX: 0000000000100000 RCX: 0000000000100000
> >> steamdeck kernel: RDX: 0000000000000100 RSI: 0000000000100000 RDI: 0000000080200000
> >> steamdeck kernel: RBP: 0000000000000100 R08: 0000000000000000 R09: 0000000080200000
> >> steamdeck kernel: R10: 0000000000000004 R11: ffffffff9fa03ad0 R12: 8000000000000163
> >> steamdeck kernel: R13: 0000000000000000 R14: 0000000080200000 R15: 0000000000000000
> >> steamdeck kernel: FS:  0000000000000000(0000) GS:0000000000000000(0000) knlGS:0000000000000000
> >> steamdeck kernel: CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> >> steamdeck kernel: CR2: ffff888000000413 CR3: 000000020ed02000 CR4: 00000000000000b0
> >> steamdeck kernel: Call Trace:
> >> steamdeck kernel:  <TASK>
> >> steamdeck kernel:  ? early_xdbc_parse_parameter+0x32c/0x360
> >> steamdeck kernel:  ? setup_early_printk+0x4f5/0x520
> >> steamdeck kernel:  ? do_early_param+0x44/0x70
> >> steamdeck kernel:  ? parse_args+0x233/0x420
> >> steamdeck kernel:  ? __pfx_do_early_param+0x10/0x10
> >> steamdeck kernel:  ? parse_early_options+0x29/0x30
> >> steamdeck kernel:  ? __pfx_do_early_param+0x10/0x10
> >> steamdeck kernel:  ? parse_early_param+0x64/0xc0
> >> steamdeck kernel:  ? setup_arch+0x542/0xbc0
> >> steamdeck kernel:  ? _printk+0x6b/0x90
> >> steamdeck kernel:  ? start_kernel+0x66/0x9a0
> >> steamdeck kernel:  ? x86_64_start_reservations+0x24/0x30
> >> steamdeck kernel:  ? x86_64_start_kernel+0xcc/0xd0
> >> steamdeck kernel:  ? common_startup_64+0x13e/0x141
> >> steamdeck kernel:  </TASK>
> >> steamdeck kernel: ---[ end trace 0000000000000000 ]---
> >> 
> >> Signed-off-by: Umang Jain <uajain@igalia.com>
> >> ---
> >>  drivers/usb/early/xhci-dbc.c | 8 ++++++++
> >>  1 file changed, 8 insertions(+)
> >> 
> >> diff --git a/drivers/usb/early/xhci-dbc.c b/drivers/usb/early/xhci-dbc.c
> >> index 41118bba9197..699a9ac6d6c3 100644
> >> --- a/drivers/usb/early/xhci-dbc.c
> >> +++ b/drivers/usb/early/xhci-dbc.c
> >> @@ -76,6 +76,14 @@ static void __iomem * __init xdbc_map_pci_mmio(u32 bus, u32 dev, u32 func)
> >>  
> >>  	sz64 = 1ULL << __ffs64(sz64);
> >>  
> >> +	/*
> >> +	 * Check that size does not exceed fixed boot-time mappings
> >> +	 * dictated by NR_FIX_BTMAPS. early_ioremap() will WARN_ON()
> >> +	 * and not map memory in those cases.
> >> +	 */
> >> +	if (sz64 > (NR_FIX_BTMAPS << PAGE_SHIFT))
> >> +		sz64 = NR_FIX_BTMAPS << PAGE_SHIFT;
> > 
> > You are bounding the size here, but does this mean that the hardware
> > itself is just broken and should be fixed up to properly report the
> > correct size?  Does this hardware actually have a debug controller?
> 
> Hmm, to answer your first question as far as I have read, the size is
> inferred from PCI config with base address. My understanding here is
> that
> for early xhci debugging - the page mappings are fixed, so we need to
> bound the size for early_ioremap().

So the device is providing the wrong size here?  Why is this an issue
only for this hardware and not other hardware?

> The second answer is, yes the hardware has debug controller. We can
> configure it after boot
> and it works with USB3.1 Superspeed cable with host:
> 
> (on target)
> (A+)(root@steamdeck ~)# echo enable  >
> /sys/devices/pci0000:00/0000:00:08.1/0000:04:00.3/dbc
> (A+)(root@steamdeck ~)# dmesg | grep DbC
> [  366.276124] xhci_hcd 0000:04:00.3: DbC connected
> [  366.637223] xhci_hcd 0000:04:00.3: DbC configured
> 
> (on host)
> [114288.650481] usb 4-2: new SuperSpeed USB device number 55 using
> xhci_hcd
> [114288.662907] usb 4-2: New USB device found, idVendor=1d6b,
> idProduct=0010, bcdDevice= 0.10
> [114288.662935] usb 4-2: New USB device strings: Mfr=1, Product=2,
> SerialNumber=3
> [114288.662945] usb 4-2: Product: Linux USB Debug Target
> [114288.662953] usb 4-2: Manufacturer: Linux Foundation
> [114288.662960] usb 4-2: SerialNumber: 0001
> [114288.666320] usb_debug 4-2:1.0: xhci_dbc converter detected
> [114288.666474] usb 4-2: xhci_dbc converter now attached to ttyUSB0
>  
> 
> > 
> > And what changed to cause this to start complaining?  Does it fix a
> > specific commit?  This hardware has been around for a long time, did we
> > mess something up in the kernel for it recently?
> 
> This fix/patch is only for early xdbc. As mentioned above, it can be
> used/configured after boot.
> So yes, the hardware has been around but I don't think that early xdbc
> capability was investigated at any point in time. I don't see recents
> commits early xhci as well so maybe it was hidden all this time.

Ok, but it is good to figure out what is "wrong" here that this hardware
is causing this warning to happen, yet other hardware does not.  Either
the kernel has always been wrong (totally a possibility), or this
hardware is reporting invalid ranges.  Figuring out which is true would
be good to figure out here.

thanks,

greg k-h