Re: kern/58643: bus_dma(9) fails to bounce misaligned inputs requiring extra segments

[gnats-admin%netbsd.org@localhost]

The following reply was made to PR kern/58643; it has been noted by GNATS.

From: Jason Thorpe <thorpej%me.com@localhost>
To: Taylor Campbell <riastradh%NetBSD.org@localhost>
Cc: "gnats-bugs%netbsd.org@localhost" <gnats-bugs%NetBSD.org@localhost>,
 "netbsd-bugs%netbsd.org@localhost" <netbsd-bugs%NetBSD.org@localhost>
Subject: Re: kern/58643: bus_dma(9) fails to bounce misaligned inputs
 requiring extra segments
Date: Tue, 27 Aug 2024 06:37:02 -0700

 > On Aug 25, 2024, at 1:49=E2=80=AFPM, Taylor R Campbell =
 <riastradh%NetBSD.org@localhost> wrote:
 >=20
 > Attached patch attempts to address the issue for x86.
 
 Bounce buffer allocation (and resource allocation in general) is =
 definitely something I=E2=80=99ve never been totally happy with in =
 bus_dma.  It may be worth revisiting the issue entirely (resource =
 exhaustion in IOMMU-having systems is a concern, too).  But I don=E2=80=99=
 t particularly like having to allocate the resources lazily, either, =
 because it=E2=80=99s then possible to get into a situation where the I/O =
 may never take place.
 
 In any case, yah, your analysis is correct and your patch is =E2=80=9Cfine=
 =E2=80=9D modulo the memory cost issue.  I=E2=80=99m not particularly =
 stoked to add another flag to opt in to the behavior, because really any =
 driver that has this constraint needs this and the information is =
 already available in the arguments used to create the map.
 
 -- thorpej