Date: Wed, 19 May 2021 11:10:08 -0600 From: John Nielsen <lists@jnielsen.net> To: Jung-uk Kim <junguk.kim@gmail.com> Cc: "freebsd-acpi@freebsd.org" <freebsd-acpi@FreeBSD.org> Subject: Re: How to properly locate/parse ACPI table from kernel module? Message-ID: <A63CF8E2-F185-495A-99C1-421781356D8B@jnielsen.net> In-Reply-To: <6e1f13bd-dc8f-6b30-2b62-2841deea8781@gmail.com> References: <BCFCDCE9-F169-4C1C-AD0C-FDE511C0BC11@jnielsen.net> <6e1f13bd-dc8f-6b30-2b62-2841deea8781@gmail.com>
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> On May 17, 2021, at 2:27 PM, Jung-uk Kim <junguk.kim@gmail.com> wrote: >=20 > On 21. 5. 17., John Nielsen wrote: >> I=E2=80=99m not much of a kernel programmer but I=E2=80=99m trying to = maintain/improve the isboot module, which allows booting directly from = iSCSI by reading the iSCSI Boot Firmware Table (iBFT), bringing up the = interface with the details specified therein and connecting to the = specified iSCSI target before trying to mount root. >>=20 >> I=E2=80=99m not the original author but as the port maintainer I am = hosting the code here: https://github.com/jnielsendotnet/isboot >>=20 >> I have a test system where the module loads but fails to find the = iBFT. I reviewed the iBFT code and realized it has a bunch of magic = numbers mixed in with some random memory diving. If I=E2=80=99m reading = it right (see = https://github.com/jnielsendotnet/isboot/blob/master/src/ibft.h#L37 and = https://github.com/jnielsendotnet/isboot/blob/master/src/ibft.c#L521), = it looks like it scans all of the (kernel?) memory between 512K and 1M = in 16-byte increments looking for one beginning with the string = =E2=80=9CiBFT=E2=80=9D, which if it finds will be used as the offset for = reading the table. I don=E2=80=99t know where the 512K and 1M values = came from or if they are correct, but I do have a system where that = method does not work. >>=20 >> IIUC, the iBFT is an ACPI table, and it seems like using ACPI to find = it would be safer and more reliable. So my question is: how does one do = that? Are there other places in the kernel code that do this sort of = thing that I could use as a model? Any gotchas I should know about as a = (less-than) novice kernel programmer? >=20 > You may use AcpiGetTable() and AcpiPutTable(), e.g., >=20 > status =3D AcpiGetTable(ACPI_SIG_IBFT, =E2=80=A6); Thank you (and Andriy) for your responses. Good to know that = ACPI_SIG_IBFT is already defined in the upstream headers. What is the second argument (=E2=80=9CInstance=E2=80=9D) of = AcpiGetTable()? Is it just an offset in case there are multiple = instances of a given table type? Also, when/why should AcpiPutTable() be used? Thanks! JN
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