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Sun, 02 Aug 2026 22:05:03 -0700 (PDT) From: Alec Hall To: pepemontfort@gmail.com Cc: jikos@kernel.org, bentiss@kernel.org, dmitry.torokhov@gmail.com, andfed.net@gmail.com, linux-input@vger.kernel.org, linux-kernel@vger.kernel.org, Alec Hall Subject: Re: [PATCH v2] HID: input: read battery capacity from its actual report offset Date: Mon, 3 Aug 2026 01:04:57 -0400 Message-ID: <20260803050458.40134-1-signshop.alec@gmail.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260728201309.776026-1-pepemontfort@gmail.com> References: <20260728201309.776026-1-pepemontfort@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On Tue, Jul 28, 2026, Jose Villaseñor Montfort wrote: > Store the battery field's offset within the report at setup time and use > it when querying, so the capacity is read from its real position. I can reproduce this on a second model and a second transport, and the report descriptor explains it exactly. Magic Trackpad 2 [Lightning] (05ac:0265) over Bluetooth, 7.1.5, battery report 0x90: Field(0) Usage(3): Power.Good, BatterySystem.Charging, BatterySystem.FullyCharged Report Size(1) Report Count(3) Report Offset(0) Field(1) Usage(1): BatterySystem.AbsoluteStateOfCharge Report Size(8) Report Count(1) Report Offset(8) So the capacity lives at report_offset 8, i.e. buf[2], and buf[1] is the flags byte -- the same layout you found on the 0324. Sampling sysfs capacity and a raw HIDIOCGINPUT of report 0x90 side by side, from the moment the trackpad reconnects over Bluetooth: 00:50:36 sysfs=0 raw bytes: 90 00 4f 00:50:42 sysfs=0 raw bytes: 90 00 4f [...] 00:51:35 sysfs=0 raw bytes: 90 00 4f 00:51:40 sysfs=79 raw bytes: 90 00 4f 00:51:44 sysfs=79 raw bytes: 90 00 4f The device says 0x4f (79%) in byte 2 throughout and never changes. For 64 seconds the kernel reports 0%, which is buf[1] with the trackpad discharging (flags 0x00); your 3% is the same byte with the thing plugged in (flags 0x03). At 00:51:40 the first battery report is parsed, status becomes HID_BATTERY_REPORTED, and reads switch to the correctly parsed value. Only the path changed, not the device. I checked the offset semantics the patch relies on and they hold: - struct hid_field documents report_offset as "bit offset in the report", and hid_report_raw_event() strips the report ID before parsing (cdata++/csize-- when the enum is numbered), so the offset is relative to the data after the ID. 1 + report_offset/8 is the right expression. - The +1 also survives unnumbered reports, which was my first worry. usbhid_get_raw_report() states "Byte 0 is the report number. Report data starts at byte 1" and offsets the buffer when the number is 0; hidp_get_raw_report() likewise puts the report number in data[0]. Byte 0 always holds the ID. Might be worth a line in the commit message, since it is the obvious objection. - Growing the request from a fixed 4 to max(len, 4) is safe on Bluetooth: hidp_get_raw_report() copies min(skb->len, count) rather than failing on a size mismatch. Two things I would raise, neither a blocker: 1. hidinput_setup_battery() only receives the field, so bat->report_offset is the offset of usage 0. hid_input_field() extracts usage n at report_offset + n * report_size, so a device that declares the state of charge as a non-first usage of a multi-usage field would still read the wrong byte. Not the case on either of our trackpads -- above, the capacity is the only usage in its field -- but the flags field right next to it is Size(1) x Count(3), so multi-usage fields are entirely normal in this same descriptor. hidinput_configure_usage() already has usage_index, and the feature-report loop has its index too, so storing field->report_offset + usage_index * field->report_size would close that without much churn. That said, I would not want to load scope onto a fix that is already correct for every device we know of and is headed for stable -- if you would rather keep this one minimal, I am happy to send that refinement as a follow-up on top of yours instead. 2. offset/8 plus a single-byte read assumes report_size == 8 and a byte-aligned offset. True here and true of the old hardcoded buf[1], so this is not a regression, but a device with a 16-bit or bit-packed capacity would read garbage silently. A guard that falls back rather than mis-reading might be worth it. One observation that supports your "permanently 3%" case: the STATUS branch of hidinput_get_battery_property() does not just query, it also writes the queried value into bat->capacity and sets HID_BATTERY_QUERIED. So on a power supply that nothing ever reports, the flags byte is cached and stays, rather than being re-read as a transient. That is consistent with the second supply on your 0324 sitting at 3% indefinitely. For what it is worth on the duplicate-power-supply side: my 0265 over Bluetooth declares AbsoluteStateOfCharge only in report 0x90 and gets a single power supply, so the duplicate pair looks specific to the 0324 descriptor rather than common to Magic Trackpad 2. Reviewed-by: Alec Hall I have not built a patched hid.ko, so no Tested-by from me -- the above is the unpatched behaviour plus a code read. Happy to test a v3 here over both USB and Bluetooth if that is useful.