efi_capsule_write(9)

efi_capsule_write(9) 💖✨

Okay so bestie, welcome to the Kernel API Manual corner where today’s main character is efi_capsule_write. She’s here to store the capsule binary and hand it off to the efi_capsule_update() API. Slay.

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✨ NAME ✨

efi_capsule_write — store the capsule binary and pass it to efi_capsule_update() API

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🌸 SYNOPSIS 🌸

ssize_t efi_capsule_write (struct file *file , const char __user *buff ,
size_t count , loff_t *offp );

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🎀 ARGUMENTS 🎀

• file — file pointer. literally the file pointer, bestie.
• buff — buffer pointer. the buffer in question.
• count — number of bytes in buff.
• offp — not used. iconic “not invited to the function” energy 💅

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💅 DESCRIPTION 💅

Expectation:

• A user space tool should start at the beginning of capsule binary and pass data in sequentially.
• Users should close and re-open this file note in order to upload more capsules.
• After an error returned, user should close the file and restart the operation for the next try otherwise -EIO will be returned until the file is closed.
• An EFI capsule header must be located at the beginning of capsule binary file and passed in as first block data of write operation.

lowkey important: that -EIO situation stays until the file is closed, so don’t just keep retrying into the void, okay? 🫶

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🦋 SEE ALSO 🦋

• Kernel file ./drivers/firmware/efi/capsule-loader.c
• efi_free_all_buff_pages(9), efi_capsule_setup_info(9), efi_capsule_submit_update(9), efi_capsule_release(9), efi_capsule_open(9)

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Source/version vibes: ./drivers/firmware/efi · date: October 2026. bye bestie, go write that capsule correctly 💜🎀